With regards to Justin's IDI, yeah, sure, he made a few more HP. But he did a /lot/ of mods to make that happen, that don't apply to 'normal' IDIs. Also, he's pretty close to sea level(enumclaw, WA = 761' above sea level), vs 2K for me or 1440 for thewespaul
Also, pretty sure that he smoked quite a bit. Fine for the dyno, not so much for driving.
Remember that smoke = high EGTs, at least in my experience.
As far as deciding to stay NA... I was all about that myself for a few years. I tried a bunch of mods, tuned it, messed with it etc.
Put the turbo on and I will /never/ go back. The turbo just makes it *so* much better, it's not funny.
Below 45(unloaded), you don't see much difference in acceleration. Above 45, it makes a difference. Hauling, it makes a difference.
And don't even think about going 70-80 with a decent load on, NA. You'll be maxing it out the whole way. Turboed? Just give it more throttle and take that hill at speed!
Exhaust wise... I've tried it. I whacked the muffler off when I had a NA truck. No real difference. Sounded a /lot/ louder, though.
Ended up putting a Thrush Welded muffler on it at that point, which made it bearable again.
Power... well, no real difference.
With my turbo rigs, it's different. It's actually reasonable to drive it straight-piped, without getting hearing loss. The turbo quiets it down quite a bit, especially on acceleration or any time you have boost(so it's loudest when decelerating).
You can also throw a nice glasspack on there and quiet it down a lot. Ended up with that on my 88, and it removed a lot of noise and, again, no noticeable power loss.