Check6, I believe the stall speed will increase with altitude (above about FL200). Relates to reynolds number, and the compressibility of the air. (lots of "I think"'s in previous statement)
Most large modern jets have thrust limits that are similar (in alt) to buffet limits. Result being, if you do drag your butt up to max CRZ level (at 300'/min or less), flogging the engines all the way, then you have to sit there for the next 2 hours hopeing for a smooth ride, as Vs, and MMO, are looking pretty friendly with each other.