Ok since originally posting this, I've switched to an Exedra Max, I'll do some measuring tomorrow and update the first post.
What I do know though, is that the 240/55 vs 240/50 for me changed nothing regarding the speed of the steering. I don't have an explanation, but the Exedra feels similar to the cobra despite the geometry change.
Changing the steering bearing, top and bottom, did greatly improve feel and feedback from the front end, which increased my confidence in flicking the bike around.
Stable somewhere north of 155mph so I'll update the original post, because I now know the slightly sketchy feel I had was all the worn out steering bearings.
Getting to the edge of the Exedra Max is awesome fun. Seat off saddle, elbows loose, head and upper body down and inside I was able to take a 270 degree 30mph ramp at just under 70 without dragging any hard parts. My car starts squeeling and understeering at like 54-55. I'm sure a real sports car would be much faster around it.