I would agree it's a chassis class. But I would also submit that it has little to nothing to do with the amount of seats. I don't see many people jumping to build Ford EXP's, Fiero's, Mercury Capri's or any other 2-seater on a shit platform.

The Lotus, Miata and RX-8 all have one thing in common - great suspension design. Is that 'typical' of cars with 2 seats? Maybe more so than 4 seats but there are plenty of 4-seat sporty cars...and plenty of 2 seat turds. If you want to regulate something, regulate suspension design.

As with any racing class, you need to look at suspension design and potential for power. Pick the weapon that combines your perception of the best combination of attributes.