Yep the I20 is a hard boat to beat in predominantly light air venues. We've been sailing a mixed fleet of F18's/I20's and the real kicker is the heavier teams on the F18 (over 370lbs) are just generally not competitive with the 20's, which isn't fun after a while .
We actually use the SCHRS rating for the Nacra 20 One Design which is also incorrect (the correct value is 0.957, there is an error with the jib size in the SCHRS table). Anyway this computes to 2m41s an hour, which isn't trivial given that in many conditions a top F18 team can beat a top Nacra 20 team.
Anyway the N20 modified with a lifted rear beam and updated sail plan is still a real weapon. They are long in the tooth though and the F20c is a very nice replacement.