It'll cause it to boil at a lower boiling point, and that will slow the reaction down like Rh says.
sealed reactions are for dummies, the same can bee acheived with a Push/Pull apparatus.
There's so many thing's that could be done with that to change pressure required to move the fluid, not just larger amounts of fluid...
Is there a critical reflux temperature in this rxn where ephedrine starts decomposing or otherwise takes damage?
There's some documents I downloaded regarding pressurized HI/P reactions in industry, Ill probably find something in there.