At the higher end of beachcat performance, it really comes down to the skipper
On my H-20, I have beaten N-20s, and barely beaten H-16s
Back when we had some top-caliber guys sailing a variety of boats around here, the old single-trap Tornados, P-19s, H-20s and N6.0s were very close. A spin-equipped Marstrom Tornado will walk on a H-20, but usually ends up pretty close to a N-20 (assuming an Olympic skipper isn't on board). If you are a very good skipper, and they aren't, you can stay close upwind on a H-20, but they are pulling away. Downwind, with no chute, you're dead.
If you're looking for an ultimate ride, I would lean towards the Nacra 20, because they are cheaper with less expensive parts, and almost as fast as the T. A Marstrom T is an absolute work of art, but you pay for it, and you need an artist to fix it. An H-20 with a snuffer kit is a heck of a performance bargain, and a BIG step up from an 18. You could get one to train on and sell it for what you paid for it while you wait for that good deal on the ultimate ride