The Xstream's actually specify a slightly
lower intermediate pressure of 123psi. I believe some of the older Poseidons used a higher one. I don't know what the XT's call for, but I assume it's probably the more standard 'about 135psi'.
In the end, I've slapped Xstreams on first stages putting out 140psi and had them work with no problem, so it isn't a showstopper. Tuning the Xstream second stages to whatever pressure the first is running at is also very easy.
That said, the
actual issue, is a safety one. Xstream second stages being a "downstream" valve design, require some type of overpressure valve on the first stage. Xstream first stages have them built it, but if you are using a different first stage, you have to add one yourself. Cost all of $5:
Over-Pressure Relief Valve (User Adjustable)