Changing over to the round tops wont give you any signifficant power gain. What it will do is enable you to better tune your engine. The flat tops were a stop gap measure to work with the addition of more emission control gagetry. They were used between the carbed 240 and fuel injected 280. The earlier style carbs are better and less troublesome to tune.
I think your likely to lose performance with roundtop. Here's why:
Nominal bore is the same so they should flow the same amount of mixture give or take a smidgeon.
1. Powerjet will assist performance on throttle-on transients.
2. Fuel bowl and float centered on jet will provide uniform fuel level control and mixture control under cornering, acceleration. SU themselves adapted this design on their later model carbs.
3. Larger air piston provides more precise control of needle position in jet for improved mixture control.
There are stories out there of successful racing flattops including a 260 that took SCCA C-stock national champion two years running some years back. Ain't gonna do that with no crap carburetor!
Got no beef with you guys about round-tops being easier to tune/troubleshoot. Can't get much simpler than a round top. Perform **** good too. (If you wanna get sophisticated you can start playing with needle taper.)
Ya gotta love a flat top. Speaking of Round Tops...... There is a complete '67 1/2 1600 Roadster being parted out..... Now I don't think the round tops will fit the later Z's but if you know anyone looking for parts drop me an e-mail.