For what its worth here's my 2 cents.
How intensive was your mockup and was your helmet resting DIRECTLY on the bar? If you just set the seat in and moved around you're not necessarily in the bolt-in position. Also, did you consider the very restricted movement of the 6-point harness you're installing? You'd be perfectly centered and hardly able to move when cinched down.
You have more room than many so if you considered all the other items above then I'd say its a factor of the aftermarket seat and bracket setup which others don't have. I would evaluate the ability to offset the seat or frame along with the necessary strengthening of any seat mounting points. I agree removal of the frame is a bad idea, autocross is already painful enough in these cars because I can't really see the right front wheel properly unless I'm crunched against the roof.
I don't see much reason/benefit to redoing the bar, I would evaluate the harness and seat setup and where exactly it places you in the vehicle.