Take a look at entries in the garage here, and on Fuelly:
EcoModder Fleet list - EcoModder.com
Acura RSX MPG - Actual MPG from 374 Acura RSX owners
It looks like in the hands of the right driver, in perfect conditions (warm air, no A/C, low speed highway cruising) you can probably expect to average upper 30's. In the hands of most drivers, upper 20's are realistic.
My guess is that much of it is from gearing; the gearboxes on these cars have notoriously short top gears. You could probably swap in the 5 speed from an Accord K24, or perhaps just use the final drive in an RSX 6 speed.
That said, I'd love to drop a K20 in my Insight. Due to the light weight and low drag, 50mpg with 200hp+ (in a sub-2000lb car) on tap is realistic.