I think if there were a current Mame Core the performance would be really bad compared to the earlier Mame Cores..
I just finished updating to the Mame 2010 Core... 37gb!! and I can already see there is performance differences, the game Bust A Move stutters when played on Mame2010.
So you have to weigh more Roms against performance.
The Cave CV1000 now has a recompiler, which could possibly make the games from that system to run on the WiiU. That's what I'm missing mostly.