performer heads are much better. I consider GT40's not so much of a performance head, but rather just not as bad as the stock castings. They are good for a mild engine but they do leave a lot to be desired. If they're the same price, I would definately go with the edelbrock performers. Do a google search on Stan Weiss cylinder head flow data, very comprehensive list of flow vs. valve lift for just about every head made.
Somebody in this forum wrote that y303 is basicly same as edelbrocks older performer? I really don´t know this as a fact. It is really hard to tell the difference, if both are aluminum and don´t have to think of weight difference... At least here y303 GT40 heads are much cheaper than edelbrocks (new). If I´d be thinking changing my -69 heads, Id go to AFR , I think, but maybe I´m just brainwashed thatway... ???
I took the online data and averaged flow for the Edelbrock Performer and GT-40X heads and created the chart below. I compared the flow numbers to a stock 289 head.
GT-40X Heads flow 24% better than stock.
Perfomer 5.0 Heads flow 30% better than stock.
This is how they stack up against the competition. These values are average Intake and Exhaust accross a lift (.200 - .500). They are sorted in order of highest average flow. I didn't have complete data for these from (.100 - .600) so this could affect the orders, but not by much.