Sledge is a modified custom build F82R, carbon I think, where as a F28 is a gelcoated glass production boat, slightly longer, but likely a bit heavier.
Tri's generally outperform similar cats in the light due to being able to induce a little bit of heel and tack more easily, and will have slightly less wetted surface area. However the cats catch up once the breeze comes up a bit.
You'll likely be racing the 28 R on the water, if it's hull flying weather you may have a sniff at the 31's depending on how race orientated they are