Hi EBS,
I have a number of Poseidon Jetstreams on which I've had installed SeaCure Sport model II (blue) mouthpieces. Officially SeaCure says their product won't fit Poseidons, unofficially it CAN be done, but it's not easy.......it takes a LOT of strength while at the same time slowly easing it on the 2nd stage orfice. The LDS owner is an ex-weight lifter and very technically proficient, and he got the knack of installing them after we trashed the 1st $30 SeaCure mouthpiece (it split while being forced on the 2nd stage)......note, the yellow insert HAS to be removed first, and it has to be eased on both forcefully, yet carefully......by the way, the other dive shops who've seen my regs have all been amazed to see them installed......one even called SeaCure to let them know their product CAN be made to fit Jetstreams, he was that impressed!
I don't know if the Xtream orfice is the same size as the Jetstream though. I've been thrilled at how well the SeaCures have performed on my Poseidons...I can't imagine diving without them.
Karl