The plates that came with the clutch were identified as Surflex plates by the guys at Moto Rapido, so were the ones with the clutch, it was stripped down and cleaned up before refitting, and checked for wear (virtually none

) Your's are probably the same sintered type of plates.
The slip when slamming shut is very slight, at similar speeds, and I was feeling for it at the lever, which you wouldn't be doing on the back straight. If I wasn't feeling for it, I'd not have noticed it.