I don't think that, programmed like it was in this case, Genesis could handle more animation sprites without noticeable slowdown... Remember that NeoGeo 68K is running @12MHz and Genesis's @7.67MHz.
NeoGeo also uses faster rom chips, so, even if NeoGeo cartridge was a "regular" one like Genesis's or SNES's it would still be faster.
Furthermore, the NeoGeo cartridge slot is so much powerful, and using all the components inside those beast-freakin-expensive-huge cartridges it produces a phenomenal bandwidth... There are brutal differences comparing it to Genesis side of things...
A 32MBit cartridge could provide the cut out bonus stages and all arcade samples (with better quality than the 24MBit port maybe); but not much more than that.