GT40 heads

If you leave the gt40s stock, probably not anything depending on what you have done with the e7s.

ported vs ported, I have seen maybe 15-20hp

Just beware of the larger combustion chambers of the early gt40 (non-P) heads. I kinda fell into that on my car, and wound up with absurdly low compression. The P heads have a chamber size about that of the e7 heads, but they have a goofy spark plug angle that wont clear most headers, at least without denting. Since your current heads are already ported, you would be best to just save up and get the trickflows, or whatever you want. Edelbrocks Performer/RPM heads (170cc) are common used and have excellent flow numbers too.

You can get 100 HP or more above E7s with a good set of aluminum heads. Stock vs stock GT40 vs. E7s are worth about 30 HP max. I agree with the other poster that you might not gain much depending on what exactly has been done with your E7s. Of course the actual gain in HP that you will get also depends a lot on all the other engine and exhaust components.

Edit: I'm running GT40s port matched only in my '89 Mustang (basically a Cobra top end plus a Comp XE270HR-12 cam). I noticed a huge difference between this engine and the stock 302 HO motor. However, I have all the Cobra intake parts, 1.7 roller rockers, shorty headers and an X pipe with no cats - so there are no stock HO components to limit the breathing. I guess I'm saying the GT40s can make a definite improvement over E7s seat of the pants if the rest of the intake and exhaust can support them.