My suggestions come on several levels, the first being to adopt a realistic sense of what these old trucks can do... and cannot do. You are dealing with 40 year old tech here. The power density of an IDI engine simply isn't equivalent to anything available today. Most diesel light truck engines of today can make more power stock than ours can do even highly built. It's a simple fact of physics. 11,000# is a load for an IDI truck and you are probably running over the GCVWR at that (of course I've never done that myself ( : < ). Bottom line, realize you don't have a budget DMax or Common Rail Cummins there and it never will be one unless you swap one of those engines into it (in fact, a Cummins swap is probably your most practical and cost effective power answer).
My first practical suggestion is to find out where you are now. Throw the truck onto a dyno and see what you are getting. If you are in the 195-220 rwhp range, there isn't much left to get that won't take either loads of cash to get and/or start cutting into reliability. "Boost" is just a number. Pressure is resistance to flow and sometimes engines with more boost are flowing less and making less power than those with a higher boost number that only exists because the air can't get thru the engine. Anyway, if you get some good dyno numbers then you will know better where you are and maybe get some hints on what to do next. If you are in that range above and you really gotta have more...maybe you need a newer truck or an engine swap.
I have hauled 11,000 pound many times over the past 27 years of owning this truck and I know it makes around 200 hp at the rear wheels most times. It used to cross the Rockies at 18,500 GCVWR and 40-45 mph in the slow lane, rolling smoke and running 900-1100 EGT. Many times I had to back off to keep EGT in a safe range. Well, I knew that was about all I could reasonably expect, so I adjusted my driving style and schedule to suit the capabilities of the truck. Another think I did when traveling was to leave some stuff at home. I used to think I needed to have my good home water with me, so I carried around 800# of water. What a moron! You'd be amazed at what a performance increase comes from dropping 800# of weight!
Going back to boost, another suggestion is to hook up a drive pressure gauge, with is nothing more than an exhaust pressure gauge at the turbo (google "what is Drive pressure?" to get a more complete explanation and save my fingers). Optimally, boost and drive pressure are the same but typically drive pressure is higher than boost. The higher drive pressure is vs boost, the more restriction and backpressure is developed and at higher EGTs.
What sort of exhaust do you have? The most important part is up front.. the first three feet or so. Unfortunately, there isn't room for much larger than about a 3-inch downpipe and that's what is calculated by the folks that size the turbos to the engine, but if your turbo outlet is at least 3-inch, smooth and unkinked for about the first three feet or so, you are doing good. If it's a homemade, crimp-bent, dented or jagged pipe, there is room for some gains. A bigger system after that point doesn't gain all that much but you want a nice flowing system. Years ago I put a larger system on after the Banks mandrill bent downpipe. I did see the boost come up a few PSI but it didn't yield any power. Conversely, on a trip one year the original Banks system fell off (after ONLY 15 years) and I had to have a muffler shop do a crimp-bent system on the road to get home. Boost dropped about 2 psi and EGTs were higher but I really couldn't feel much of a drop in power.
For towing with our old tech engines, my opinion has long been that a properly-sized NON-wastegated turbo is better. You don't get that off-the-line acceleration snap that you do with a wastegated turbo, but non-wastegated are often more efficient and generate lower EGTs and drive pressure than wastegated turbos. Those steady state, long term loads are what kill you EGT-wise more than anything, IMO, which is why I never went to a wastegated turbo.
I think you could probably stand a bit more boost... certaily a bit more than 10 psi but no more than 15 unless you are intercooled. Most aftermaket systems are designed to make around 10 psi at full power. Why you don't have it ??? I don't know exactly. Wastegate adjustment? If you are close enough, seek out Mel and have him tweak your timing and wastegate.
Again, find a dyno and make a least three good runs with the engine fully warmed up. Then you WILL know.