We have a 2006 HR Admiral SE ('05 W-20 chassis) with a 5-speed Allison. We are unaware of the 6-speed Allison transmission before the 2007 model year. Recently Banks installed the cat-back version of their single-exhaust system on our rig, at 50K+ miles. We chose the less expensive version without headers for less cockpit noise and possibly less heat. We've found that the engine spins MUCH more freely above ~3700 rpm and the exhaust has a deeper, throatier (and slightly louder) sound, although exhaust noise at light throttle while cruising is largely unchanged. Pulling power on hills is much improved. We can now maintain better speed on long, hard climbs to altitude, especially in the heat of summer. The engine pulls SO much more readily at high rpm that we spend less time at high rpm and shift quicker when accelerating. [Don't know about "ultrapower" or "DYNO". Our Banks system came with its own air filter/inlet system. Here in Calif, we _need_ CARB certification. ] We're happy enough with the choices we made, and would probably make them again.
Edit1: About 1K miles ago Hendersons (Grants Pass, OR) installed the BrazelsRV re-programming of GM's ECM module. Brazels advertise 7.7% more (peak) torque from 2500-3500 rpm, and the engine does seem a bit more responsive. Although 7.7% (490 vs 455 ft-lbs) is not very much at all, given enough miles the hoped-for mileage improvement may pay for itself. For us, we now have torque increases over two different rpm ranges that mesh together nicely. Banks claims up to 17% more torque at the higher rpms. In summer, we go north and high, and hill-climbing to altitude at high rpm is the rule of the day.
Edit2: After another summer travelling north and high, we have come to better appreciate the ECM reprogramming in concert with the Banks low-restriction exhaust. Athough peak torque may be just 7-8% more, the increase over OEM at 3500 RPM is quite a bit more, 20-30%, based on published curves from Brazells. These two "improvements" have made the rig much more enjoyable to drive (especially in the mountains).
