I have to agree with you David - I've found that in mixed fleet racing, F16s and F18s are not far off each other but the F18 does seem to have the upper hand in most windspeeds.
A well sailed F16 can hang with a comparable F18 on manouvreability and the fact that it does seem to accelerate quicker, but once up to speed the F18 will usually pull away until the next manouvre or mark rounding.

Round the complete course there's very little to choose between the two though as just one mistake by either can decide the eventual winner.