Will we see a slight increase in value of FG when POE2 drops kind of like we did with D2R, but not as extreme?
Or will POE2's exchange system mitigate that to any measurable degree?

dont think so

People are pretty skittish about non console trades in POE unless you're a whale; if it somehow did become popular then GGG would start banning people; as long as it's obscure (e.g. current poe forum activity) they probably don't care

Unlike the Diablo franchise, d2jsp does not seem a big figure when it comes to PoE. Sure some Orbs are traded here and there, but the overall RMT market is much bigger IMHO.