I have also a first generation but expanded with 512.. 
Thanks for answers and insights, everyone. So far Alexworp3 is the only one who actually has a non-upgraded 128kB MoonSound, so I'd say it's OK to have a song or two utilising more memory. If nobody makes music that takes advantages of the extra memory, then what good is the RAM upgrade? 
But Grauw's advice is of course good.
It'll be interesting to see what kind of tunes XM artists create with this system. Because now you have a bank of good-quality samples that you can use "for free", like for example, use several copies of the same sample in many different instruments, with different envelopes and other attributes. It's a new set of limitations and possibilities, and I think the resulting music will also sound different from conventional XM modules, but also different from MoonBlaster Wave tunes. Different technical possibilities and tools should inspire different results. Also, setting the RAM limit 128k for the majority of tunes should bring out some creativity. If the musician isn't free to use hundreds of kilos of samples like in conventional XM composing, she has to do something creative with the ROM stuff.
By the way, when writing that I just noticed that I should also make it possible to use the same _custom_ samples as multiple copies, not just the ROM samples.
In case someone wants to know how far I am: not that far. I can convert, load and play XM tunes that use the Yamaha ROM samples... sort of. But most things don't work yet, including different note pitches. And I still haven't even tried implementing the software envelopes, so I don't know how much zipper noise there is. And I don't yet have the pattern data packer system I've planned, so there's just too much pattern data to do anything real. And I want this to work with 64kB main MSX RAM.