I've had this issue for a couple of years now.
It seems that almost everytime I "plant the pedal", above 5.5k RPMS through any gears (+15 spirited boosting), and then resume normal driving (shifting), I becomes more difficult to move the shifter into any gear (up/down/reverse).
After shutting down/cooling down, everything is back to normal = shifts smoothly into any gear.
It seems to only happen when there is high demand on the drivetrain.

I suspect heat. I suspect the pressure plate. Syncros (?)

I value your insights and opinions.

Addendum: '04 R53, @89k mi on the clock - I've serviced/changed the transmission fluid (2/qts Redline MTL) @ every 10k - 12k mi. I drive an average 4k-5k mi/yr). Transmission is not leaking and check the level/color every 6mo. Clutch disk replaced @75k mi - Nothing else.