If I can get 90+% of the way to orgasm with a penis length of 'x' (which easily reaches passed the posterior fornix, but not by a lot). And also 90+% of the way with my finger (which reaches the anterior fornix), would a longer length make much difference?

Don't ask me that question, please. I'm conflicted.

Easy enough to find out. Get a big carrot, put a condom on it, and see for yourself!

Ok... But I'm not sure about putting a condom on the carrot

May I ask why you are conflicted?