power vs gearing is very easy to work out though

Aerodynamics is a different ball game entirely, with far more potential to go badly wrong.
I'd think a small rear spoiler should be a fairly safe addition....but you just never know and that isnt so easily tested.
And then it could have a negative effect on the front end.

Mine definitely gets light at the rear at speed, and I'd say is a part of why it wants to traction even at 180mph ( other than the obvious ).
But traction loss aside, when on sensible tyres the car does feel fairly stable at speed. But at the same time, I'm sure it wouldnt take very much to upset it either.

I'd like to add some sort of linear pot to measure ride height or susp travel at the back as it probably is lifting and would be nice to see how much...but cant say I do those sorts of speeds very often to test it. And certainly not a chance of me doing it on the roads.