I'm sceptical about wavetables with such filesizes. Bigger isn't always better. The S-YXG50 was already pretty balanced and very close to the MU50 and MU80, even with just a 4MB wavetable.

Sadly you cannot stuff more effects into the engine, it's at its limit. Same with polyphony, even though I think 128 voices is pretty decent.

Either way, about the YMF HiEnd engine: It's the S-YXG50 with its limits removed (what we have now) plus a few calculation glitches and the SSE path removed. That's its "magic". It doesn't sound any better and it also doesn't improve any of the MU wavetables if you combine them with it.