Matty, Hugh,
Bang on.
The chassis and the grip available are the limiting factors. The comfort kit or the 2014 uprights take the three wheeler to a good place for fun driving on the road. If you want super high grip and handling on the road then triangulate the chassis, sort out the suspension and steering geometry, fit four proper modern lightweight rims, proper modern sticky tyres, get a nice smooth high revving powerful motor and paint a big 7 on the nose. Or just buy a Caterham.
If I had both the cash and the garage space I'd have a Seven alongside the Three in an instant; the perfect Ten?