The ported 308's I have on my 340 flowed 264 cfm @.500 . If I recall correctly though most OOTB Eddy RPM's are around 250 cfm @.500 depending on whose bench you're using.
And even at the "friend" price by the time I was all done having those 308 heads done I had as much into them as a set of Eddy RPM's. For all out racing that's a decent enough difference but for street use I doubt I'd notice much if any performance difference. Especially if you consider any benefit gained from losing 50lb's off the nose with the Eddy's.
If I were doing it today I wouldn't waste my time with the 308's.