Neither the Nacra 20C or the Nacra 17 has yet proven to perform beyond their Texel or ISAF handicap ratings. (i.e. Thai regatta and now the ISAF Olympic trials). Note that the Nacra 17 is rated the same as an F16 under SCHRS rating system (12 secs/hour quicker actually, but that is next to nothing)
From what I hear the N20C's rating is quite soft so I'm not not too sure about that. The N17 has not actually sailed any races yet but I would be surprised if it is slower then an F18, the foils might allow it to be pushed a bit harder in all conditions?