So outside of the tires, these bikes are pretty much street-legal.

Yeah, they have some serious high-buck suspension, and no doubt tons of titanium and carbon fiber bits to lighten them, but the great thing about Snaefell is that the terrain and distance mean they're basically high-end stock bikes with all the parts from the dealer catalog, and slick tires. Slap some DOT legal skins on, add turnsignals and a plate holder, and you have a perfectly legit street machine (Ducati 1199 Panigale R, anyone?). Yamaha will sell you 98% of this for $16K.