A twin throttle body GRA setup on a stock engine would be such overkill i'm not surprised it ran like a dog. It's like putting a 750dp holley on one. But there is no doubt in my mind the GRA twin throttle body setups are the only way to make good power throughout the rev range on a tough engine.
The impco's can be worked to deliver way more than we have any right to expect from such a small carby. They are ment to flow 425cfm (hence the name), but actually flow a little less due to some factory restrictions. Gas seems run a lot more fuel per CFM than petrol carbies, there is no way i would expect my engine to survive to even 5000rpm with a 465 holley if i ran it on fuel.
But its more of a compromise with the impcos, inorder to get good high rpm flow, you overfuel down low, the GRA setup does not have that problem. 2 425's will not work unless your engine needs 900 or so petrol cfm's to make power, plus you'll need a tunnel ram.
The impcos require the butterflies (base) from a holley to regulate the air flow. It is a great idea as holleys are plentiful and hooking up accellerator cables to a holley is never a problem. The 425 just bolts onto the base and your away!
xa-coupe will get to see just how nasty one can be at the raid
I don't know a single person running a sprintgas setup, anywhere, so I cannot make any suggestions on their performance.