I have a Mystic player in one of my games, and the psi points are such that she has decimated the battlefield on occasion by throwing several massive fireball-like effects... but then found herself on the short-end of her total after the fact (making the conservation of remaining points a focal point, and even having to go through several encounters without any psi points remaining at all.)
The issue wasn't that she had too many psi points... I think it was that because psi points were just a large number she counted down from, it made it psychologically easier for her to blow massive amounts of them on one or two big effects with little thought, and only then realizing after the fact that "Crap! There's still a lot of adventuring day left!". With spell slots... it's easier for players to see what they have in the tank and to make the connection "I probably shouldn't cast both my two 4th-level spell slots in this one encounter because that's all I have of them for the day."
What this meant from my perspective as the DM is that one large-scale battle was reduced in power and effectiveness because she destroyed massive amounts of low-powered minion enemies in short order, and then had to deal with her being stuck with essentially "cantrips" for the rest of the week (I am running 3-7 day Long Rests). It has taken some gentle nudging on my part to reminder her when she's tried to throw a second massive AoE right after the first one that blowing all of her points like that in single encounters is perhaps not her most effective use of resources.
So I don't think you need to reduce the psi total... you just need to be prepared for "nova" attacks followed by an evaporated pool in subsequent encounters if the Mystic player isn't conservative or paying attention.
