if you are running the stock intake 10-15 tops. port the stocker or upgrade the intake and you could see over 20hp from the gt40's if they are ported as well. talk to Thumper he could do some wicked things with your heads.
setup right now is the basics....C&l 76mm maf and tube, underdrive pulleys, offroad x pipe, magnaflow catback. I will be getting a cobra intake and maybe a mild cam like a trickflow stage 1 or something equivalant.
With a stock intake...no they arent going to be a huge difference. In fact...they have large combustion chambers (65.5 cc i believe) in stock form which will drop your compression around half a point. So the power you pick up from improved air flow will be partially lost due to lower compression.
I had my gt-40 irons milled 20 thou to get that compression back, then i had a performance 3 angle valve job done. I felt a noticeable improvement above 3000 rpm and it pulls hard to 5700. I did notice a slight loss in low end power with the gt-40's over the E7's.