Thanks for the reply.
Took your advice, and tried it out on a different setup; everything seemed to check out okay, aside from some out-of-range issues with my TV and the board.
Turns out – I guess this is pretty weird, but – the metal housing for the kick harness (a DB9 connector) was shorting out something on the Supergun that prevented the OSSC from getting a lock on the CPS3’s signal. As soon as I removed it, everything was hunky-dory. So weird.
Anyways, all’s good now, and the picture is gorgeous. Happy to finally have some lagless Third Strike on my cab. Cheers.