Mai shouldn't really have a problem with that match up. Their fireballs are about the same level but you can chip more with qcb+P. I'm fairly certain that Jubei's charge back forward+K grab won't get her in time after that even with a reversal so you can jump back C to punish it. I don't think there are many things he has that can punish Mai's charge down up punch either except the tip of jump back B. He doesn't have many AA options either. Use qcb+P to punish a lot of his stuff on reaction like slide and running grab. It will be easier for Mai to keep away than it is for Jubei.
Oh yeah and on GGPO, look for a Japanese guy by the handle 'mai.' in the FFS room.