Nice! Here are the Wii U-specific changes:
- WIIU/FILEIO/PERFORMANCE: Faster startup times – remove the path_is_valid() call when loading textures
- WIIU: Fix touchscreen mouse emulation
I just tried a handful of them on MAME 2003 and they mostly worked fine. Riding Fight didn't have audio but seemed to play alright. Puzzle Bobble 2 didn't have any issues as far as I could tell and had no slowdown.
I'm assuming you're using the same ROMs for both cores? Some ROMs might work that way but it's really hit or miss since each MAME version looks for different files. The MAME 2003 core is based on MAME 0.78, so you'll need to rebuild your ROMs with a MAME 0.78 DAT if you want to play them on that core. I recommend using ROMVault if you're new to ROM auditing.
I did see the slowdown on the FB Neo core. FB Neo is more accurate but much more resource-intensive, so it's just going to be slower with some games. I usually just stick to MAME 2003 unless I find a game that doesn't work.