[pokeybritches]

I think you have to add all the pressure losses up together... so the loss from the TB restriction, plus the loss from a sudden piping diameter change, plus the friction loss over the throttle plate. The 3 hp I calculated was the loss for a 2.5" diameter, 2.5" long pipe with a mass flow rate of 40 lb/min (estimate for 400 hp). A larger TB will still have loss, and your net "horsepower gain" will be the difference between the two. I need the actual dimensions to make it a little more accurate.

With a larger TB you will have less piping restriction, less of a sudden piping diameter change, and possibly less friction loss over the throttle plate (I was incorrect about this earlier... a smaller plate has a lower coefficient of drag, but there is a higher velocity of air going over it, so an overall increase of friction loss).

Here are a few more estimates for piping restriction only, with the following conditions:
14.7 psi pre-TB pressure (TB positioned before the supercharger)
40 lb/min mass flow (~400 bhp)
TB is 0.5" longer than it is wide

2.5" diameter, 3" long = loss of 0.140 psi, 3.82 hp

3" diameter, 3.5" long = loss of 0.0687 psi, 1.87 hp

3.5" diameter, 4" long = loss of 0.0377 psi, 1.03 hp

With the VF kit and the TB positioned post-supercharger, 8.5 psi of boost (23.2 psi pre-TB) and 40 lb/min mass flow:

2.5" diameter, 3" long = loss of 0.0886 psi, 1.53 hp

3" diameter, 3.5" long = loss of 0.0434 psi, 0.74 hp

3.5" diameter, 4" long = loss of 0.0239 psi, 0.41 hp

So in terms of piping diameter losses only, this is a much better mod for an ESS TS kit. I think if you added all the losses together, an ESS kit might see 3-5 hp. Just for fun, I calculated what the TS3 would see with 50 lb/min mass flow:

2.5" diameter, 3" long = loss of 0.208 psi, 7.07 hp

3" diameter, 3.5" long = loss of 0.102 psi, 3.45 hp

3.5" diameter, 4" long = loss of 0.0557 psi, 1.90 hp

So for the straight section alone, a TS3 would see more than a 5 hp loss with the a 2.5" TB versus a 3.5" TB. When you take into account less diameter changes and friction drag over the throttle plate, it looks like a gain of as much as 10 hp is possible.

But yes, on a stock car, 15 hp for a throttle body mod is a pipe dream.