Another update.
I felt changing all 4 engine mounts to the SOS ones was too stiff with too much vibration, so I exchanged back to keeping the L and R ones stock and the FR and RR units the stiffer SOS mounts. Still some more vibration than stock, but mechanic and I both feel there is less slop in the transaxle.
The engine is running unbelievably well. I mean, holy-crap well after Ramon's rebuild at Niguel Motors. Even he was suprised at how strong it is running. Other NSX factory techs over the years have commented on how they could tell certain C30 motors were 'stronger' than others for whatever that's worth, and this one has been at the far end of the strong scale ever since new.
However, since I've started beating on the car hard since the rebuild, we've noticed a rattling from the tranny, and it seems it's time for a new box. Car was originally in the snap ring range, and the case was changed in '97; now 223k miles on the original 5-speed with lots of hard shifting. In general, I've been good to the transmission, with rare missed shifts and with clutch 1 going at 92k and clutch 2 still having life on it when we changed it at 222k-- in fact pads still looked good, no chatter, and only some wear and play on ONE damper spring (which probably would have been the failure mode in another 10-20k). I've decided to go with the SOS 6-speed conversion with the NSX-R mainshaft to mate it with the higher-performing dual plate clutch. This includes the relay to lock out reverse unless the brake is depressed.
If anyone has any advice on this switch, either a lesson learned after the fact or during the change, please do chime in.