My two cents.
I just installed my Quickfit Pro R55 today and I two felt that the amount of slack on the left side of the lap belt was insufficient if using the seat belt anchor point as described by adc on page 11 of this thread. This was causing the buckle to sit off-center in order to provide adequate lap restraint.
So I decided to used the left seat-rail rearward anchor bolt as shown below. Works much better and now the buckle is centered and the lap belt is nice and snug.
Hope this helps.
And a shot of the rear. All looks great. I am pretty happy with the setup and look forward to my first track day with it next week.