As was stated here, ITBs may make very little improvement in power and not worth the expense -- on the other hand, throttle response should be markedly better, and the sound itself might be worth the expense.
MX6/Probe owners have done a lot of experimenting with the KL, and I think it's pretty clear that within the limitations of good daily driveability and cost, 170-180 whp is about the limit. But that's pretty good, and comparable to the 240, or so, a 350Z gets, w/3.5 liters. The most advanced engine I've seen is one on Probetalk that's putting down 217, and that's with stock ignition, fuel, lifters, and modest cam timing and compression. 217 is impressive in it's own right, but he's also producing that at 7100 rpm -- far below what the engine is capable of; and where the real horsepower is. I think his dyno plot underscores the fact that the engine is induction-limited.
As far as I know, no-one has built a really serious KL. That's what we are going to do. Because the Miata is very light, has a generous range of final drive gears available, and will be strictly trailered (autocross) we can try things that aren't feasible for street MX6s.
BTW, my sponsor and I have been talking about this project for years and are now taking on the boosted Miatas and MR2s -- boy, are they going to be surprised!