Racer?
Andy:
I read the same info about the "Y" winning more races and more money than the "R"s, but remember, the "Y" did not race with the big boys. It was a class winner, not overall.
The "Y"s top speed was about 170 mph. (At 1/4 scale that's 42.5 mph!) When Florence Klingensmith tried to make NR718Y go 200 mph, it came apart in midair.
Perhaps it's fortunate that the "Florence Klingensmith"s of R/C who insist on trying to make the Gee Bee ARF go 100 mph aren't sitting in it at the time.
I don't have any experience with the RCS engines, but Dick Pettit will be using one soon. Given your rpm figures though, I'd go with the 17x8. Should give better thrust than the 16x10 because of the big cowl and you should still get 70 mph top end.
Jim