In looking at dyno charts and dyno test data collected over the years, the wastegated vs non-wastegated kits develop essentially the same peak boost and power. Peak torque is nearly the same number but with the wastegated, it comes in a little sooner. The only potential downside is higher EGTs and higher drive pressure... mostly occurring at max power or under heavy loads.
For the most part, this is a bench racing debate. A PROPERLY SIZED TURBO ON A PROPERLY SET UP ENGINE (correct fuel rates) with a wastegated turbo will not produce terminal EGTs vs a non-wastegated and vice versa. A PROPERLY SIZED non-wastegated turbo will not have excessive turbo lag, but it's low rpm performance will not be as snappy as the wastegated unit. If you want to fine tune the choice- if your truck does a LOT of steady speed towing at highway speeds and with heavy loads, the non-wastegated is probably the best overall choice. It will be dialed in for that situation and deliver EGTs some hundreds of degrees lower and cooler intake temps within the RPM range the engine is tuned for. If the truck is used often as a daily driver, fun hotrod, dyno toy, drag racer and occasional tower/hauler, the wastegated will deliver the expected performance and off-line snap at the cost of higher, but not usually terminally higher, EGTs in the full-load, steady state towing/hauling situation. In the latter case, the driver can compensate somewhat by controlling the foot feed and with gear selection, plus you can add devices to help bring the temps down (a degree lower on the intake side results in a degree lower on the outlet).
Also, look at the competition engines. Do any of them use wastegated turbos? Few... none I've seen in the more potent classes. Certainly only in the low end classes of pulling. Why? A properly sized non-wastegated turbo flows more air, in and out, for lower temps, lower drive pressure etc. Of course it's at that narrow rpm range where the power is deemed to be needed.
Bottom line, the wastegated turbo is an expedient, a compromise, to improve low speed performance. A very well designed non-wastegated system can usually equal it, but it's a lot harder and more expensive to develop so....
A Long Aside-
I do NOT understand this obsession with a high boost number (well, I do but...). Higher is NOT better and lower is not necessarily worse when you look at the whole picture. It's safe to say... the guys that originally designed the turbo kits for the IDIs are probably smarter and more experienced than most of us (I've met Gale Banks and Jim Bensen-inventor of the ATS kit- and they are certainly smarter than me...) and did more testing than most of us will ever have the opportunity to do. Around 10 psi is the commonality with all the aftermarket kits and you can bet that number was not just pulled out of some gearhead's ******. It was arrived at from a lot of testing and maybe a blown-up engine, or a few.
The problem, I think, is that people are comparing DI engines (which now far outnumber IDIs) with IDIs and combined with the "bigger is better" philosophy that infects all of us, people want, and think they need, these insane high numbers so they can match the DI guys for brag gin' rights. Because of the combustion chamber design, DI engines can tolerate... actually need... more boost than IDIs. Too much airflow actually begins to interfere with the IDI combustion process a little because it starts in that tiny Ricardo chamber. With the IDI, it isn't quite as simple as filling the main combustion chamber with as much air and fuel as possible. You are injecting thru the pre chamber but air is flowing into the main chamber, which is MUCH smaller than an DI so there is less room for the mixture that goes boom. Pushing too much fuel into the pre combustion chamber (which has limited airflow) can really dampen the combustion process. You can increase the chamber size, either the main or the pre combustion, which drops the CR, but the result is "other" trouble. Less efficient combustion when the chamber is cool, hard cold starting, loss of low rpm snap, etc. You can make that tradeoff situation work for a toy but not very well for a daily driver. There is an upper limit for boost on IDIs and it's MUCH lower than DI. The PRACTICAL day-to-day boost limit for IDIs is even lower if you think about daily driving. Where that is.. I don't know. Certainly below 20 psi and probably below 15. I don't know if this has all been worked out, or if it has, it's in some obscure technical journal. I do know that the diesel industry went to DI and that's the telling industry change. They did it for a reason. Look at what power levels those DI engine are at these days, not to mention the extremely low emissions vs the IDIs. The IDI was taken about as far as it could go and the industry moved on. Practicality rules.
Back to boost. Think of it this way: Pressure, boost pressure included, can be considered the absence of flow. If you pump 100 gpm thru a 2-1/2 inch fire hose, you get about 80 psi (if I remember my basic firefighting crap correctly). You pump 100 GPM thru a 1-1/2 inch fire hose, you get nearly 200. 200 is better right. The number is higher right? Well, yeah, but it's meaningless because the actual rate of flow is the same. Any type of passage will flow a certain amount at a reasonable pressure. Pressure is the "push" that increases flow but the flow rate vs the pressure are on a curve. As the passage reaches it's maximum flow, pressure rises exponentially. There is a magic number for any passage. The flow rate increases with pressure, but much more slowly than the pressure so it's a case of diminished returns when you consider the other aspect.. heat. The more you pressurize air, the more it heats up. The more it heats up, the more it expands and the more pressure is created as it's being pushed thru the passage (not to mention higher EGTs- hotter in, hotter out). An intercooler cools the air and what happens to the pressure? It drops. Actual airflow may actually increase at that point, even if pressure is lower, because the cooler, contracted, more oxygen dense air can move more easily thru the passage. It certainly moves the intake and exhaust temps down and even with the lower pressure, the engine is actually breathing more air and making more power with lower EGTs.
The amount of power is limited by the amount of air and fuel that can be pumped into a cylinder in the correct ratio. Power is not created by increasing boost pressure, not all by itself, any more than power being created by injecting more fuel all by itself. It's all an equation and you have to look at ALL the numbers to get a good answer... the right answer.
