We're limited to 120psi by the rules (so we don't risk shrapnel
), so you can't go any higher.
What you can do is add air storage tanks (up to 4 by the rules), and your high pressure won't drop so drastically each time you use it.
You do need to consider managing your air supply and how fast and how often you can use those pistons, including precharging your pneumatics storage just before going on the field to play in a match.