Greg, 500hp is 500hp. This turbo will support the airflow for 500hp.

With everything currently available (TTPM/DD3's/Turbo) you will gain right at 200rwhp. The (250hp) SO makes about 235 at the wheels. The (235hp) SO CA model makes about 200 at the wheels. Add 200rwhp to that figure- whichever fits for your truck- and that's about where you'll be.
Lilredcummins. The currently availabe TTPM or EZ rail pressure box will make 65-70hp on an otherwise stock truck. If you have an HO, that should take you from the 275-280hp range to around 350hp. The upcoming timing/pulsewidth box has been rumored to make over 125hp by itself. That would put you in the 475hp range if all we did was add the power together. My guess is that the combination make make even more power when run together.
For example, in our testing, our Stage 2 injectors will add 75-80rwhp on a stock truck. The TTPM will add 65-70hp on a stock truck. Individually, that should add up to 145hp minimum. When put on a truck together, they make more like 170hp. This is due to the compounding effect of larger injectors and more fuel pressure.
When the new box comes out, there will be another compounding effect of pulsewidth as well. Let's think of it like this. If the injector is commanded to be open for 3 milliseconds, with 23000psi, it flows "X" fuel into the cylinder. If we raise the fuel pressure to 25000psi, now that "X" is raised as the fuel is pressed in harder, which means more flows through. With injectors, we make the holes slightly bigger, and smoother for fuel flow, meaning more can come through (among other things). So now, we're pushing more through with more pressure, and have a bigger hole, so those two compound each other. With the upcoming timing/pulsewidth box, we will now be able to increase that puslewidth from 3ms to say 3. 5ms... now we have more pressure coming through a bigger orifice for more time. See how it all add up?
Now, on to the injectors/turbo question. You have to have the TTPM if you are going to run injectors... this is because you have to have the MAP limiting (boost module) feature. This is built into the TTPM. It allows you to make any boost level without setting the check engine light and defueling the truck. You could run any level of injector, the TTPM and turbo kit and produce a decent amount of power. Most of the trucks we have done have come out in the following fashion:
HO + TTPM + Stage1 + turbo = 410rwhp
HO + TTPM + Stage2 + turbo = 470rwhp
HO + TTPM + Stage3 + turbo = 500rwhp
HO's generally make 275-280rwhp here, so those numbers are based on that starting point.
XCX, I do not see any problem with stacking the boxes logically. Until we actually test it, I can't say - but I don't think it will be a problem.
Clayton, you are correct. The 48RE is limited at 400-410rwhp in stock form. Any more power with that trans will require an upgrade. We have always used DTT with great results. Manuals require a clutch upgrade at 350hp for 6-speeds and 325hp for 5-speeds, but then they are good to go.