I am extremely pleased with the ram air installation on my RV-8. The aircraft is powered by an Aero Sport Power IO-360M1B6 with the Precision forward facing induction system. The modification to the cowling to accommodate the scoop you provided was not difficult . I used baffling rubber that was hose clamped on to the filter housing to form the seal between the scoop and the filter housing. As you can see from the photos, I made the scoop removable to facilitate cowling removal.
As to performance, I routinely get more than 1 inch of additional manifold pressure. When flying formation with a couple of other RV-8s, we are even at full throttle and 2400 RPM. As I open the RAM air valve, I can feel the acceleration and hear my buddies complaining as I pull away from them! Top speed at 75% power is 194 Kts. About 5 Kts of that is due to the Ram Air system.