well maybe we've uncovered that mystery. there's been another thread on here lately about a 4 -> 8 mb rom conversion that won't work.
perhaps that dipswitch just activates the expansion slots. the game most likely does the rom check on the main board first, but since all your roms are good, it'll pass the rom test. if it were the other way around, where you'd need the expansion, the rom test would fail, because it's missing half the roms, because the expansion isn't active via dipswitch.
useless knowledge time:
apparently there were only a handful of games based on the T-unit. I forgot that the Wolf unit was introduced not that long afterward, only the NBA Jam games and MK1 and MK2 and I think one other game used the T-unit. for the hell of it I was playing NBA Jam the other night and observed that the dipswitch for the expansion board isn't even present in that game, but it's probably because that came out before MK2. MK2 might be the only one that used the expansion...
guess the guy with the 8 mb rom conversion has a rom or two wrong. the information on the video rom locations is probably inside the 2 game roms right?