The guy who built my motor at Bennet said that I should stay with 1 and 5/8s primaries. He didn't switch to 1 and 3/4s till he was well in the 9s. Said the car always made more power on the dyno with the 1 and 5/8s. He was running a nitrous setup, so it's borderline. It may be suitable for a blower setup. Bennet said that using oversized primaries was a common mistake.
Kurt