Hi Matt,
I'm currently inside my 1985 gearbox (literally much to my girlfriend's despair) which has no output shaft front circlip. It's done 170K km (just over 100K miles) and the gears are in good order, apart from the bearings which I'm replacing.
Yes the big bearing may've been pushed forward a little and caused extra stress on the rear bearing but I wasn't experiencing what you were Matt, only a noisier gearbox with some grinding vibration through the footpegs especially noticable in 4th and 5th at speeds 60-80km/h.
The gearbox has been entered before by Prev Owner ('s) and since I've only done 15K km since owning it over 1.5 years, I don't know when the gearbox bearings were last replaced. I suspect (from what I've read on the net) that having milky trans. oil after purchasing and again on the 3rd change was probably a catalyst of wearing bearings. It still has what appears to be the original speedo cable boot, all crazied on the outer surface. I've got a new one to put back on and I'll make damn sure I seal it up with RTV on the top surface. But I digress.
Unless your gearbox has never been maintained I doubt the slipping problem is from the lack of a front circlip on the output shaft, as I said before, my one has no circlip and is good after 100K miles, only got noisy/ grindy.
P.S. Because of this I haven't gone to the length of installing a front circlip. Might be a case of last laugh - there are much more experienced crew out there!