Can't up shift during tight turn

They're not that clever!

This is quite mysterious; gears being blocked suggests the synchro units are unable to match the speeds properly. It could be that the g-force is causing the oil in the gearbox (transmission) to pile up on one side and interfering with the synchro action, but I'd have thought it was baffled against such an eventuality.

The first thing I'd do would be to check the gear oil level, and replace the oil if it's been in there for a long time - Redline MT90 is good.

Failing that, it may be a symptom of gearbox wear - how many miles has the car done?

The gears are equally worn whether the vehicle is making a sweeping increasing-radius right-hander or going left or straight for that matter. So wear would seem to be out of the equation to me. Same for lubrication too -- once the gears are wetted and carry a film of oil they are independent of the transmission oil level in the short term -- certainly for long enough to finish that single turn.

Personally, when I read the description my first thought was a broken engine/drive train mount allowing it to shift in the turn and somehow blocking the proper shift. I might even extend my guess to a problem with the hydraulic clutch where the fluid in the master cylinder reservoir was offset by G forces enough to prevent the clutch from disengaging that last little bit needed for a clean shift. Of course, it is all guesswork and is something a decent mechanic would probably find in fifteen minutes of hands-on time.

My thoughts mirror yours, John.