I've got a 2000 271 Formula with the 502efi. It runs a solid 65mph top end on the gps. When I bought it I looked at the 27 Fountain with the same power. If memory serves it ran about 68 on the gps. I don't have any experiance with superchargers, but seems like I've heard somewhere on the board about a 271 with a whipple that was running around 75mph, but that's just my memory and I could be totally off.
Good Luck and BTW my 271's for sale if your interested.