I hate edelbrock carbs. Get a holley 750, 3310 and use the RPM intake if you have the hood clearance. 70's Covettes have this issue, as the RPM intake doesn't fit under the stock hood so are limited to the performer or torker intakes.
I say you are losing at least 15 hp. The performer losses power compared to the RPM on cams over 255 to 260 degrees (advertised duration) and gets worst as the cam gets bigger.
do a web search. I found this article right away.
Dyno-Testing Small-Block Intakes - Induction Extravaganza - Mopar Muscle Magazine