[David]

Does anyone know if there are any other differences, most specifically in terms of emissions?

US Groms have the charcoal canister that Euro MSX / Groms don't have, don't know about the Japanese version...
http://www.hondagrom.net/forums/49-grom-how-s-guides/2102-charcoal-canister-removal.html

Don't know if Japanese MSX has a switchable KMH / MPH speedo and if it's a requirement in the US.

Might be more simple to find a cheap second hand one, ride it and sale it when you leave. Buy a GROM in the states when you're back.

 

[David]

That's a much better idea! Saw an original CBR250RR, the 19K RPM one go for 15000€ a few weeks ago!