I wouldn't expect much. Considering the POP intake from JWT/STillen only provides around 3 or so HP, I can't imagine a drop in doing much for you. The benefit will be that it is a reusable element. It will be the last one you'll ever buy.
I suppose if you combine the drop-in filter with the "free filter mod" you'd get a small increase. If you plan on keeping the resonator and bottom of the airbox as it comes stock, maxximtl is right- the only benefit is the reuseability. It might add a slight low growl at WOT, too... I'd save up the $$ for the CAI- you'll feel the difference. Or if you're handy, you could easily add a cone filter to the stock air plumbing, and then you'd have it for later when you make it a cold filter. Oh- the PR CAI is WAY easier to install than all the write-ups you've read lead you to believe. I'm working with them right now on a new template and bracket for the CAI- it does'nt really even require cutting the car.