I'm sure if you guys didn't abbreviate chronograph to chrono or chrony he'd have understood...too much L33t speak these days...LOL
Most stock NIB CA M15's that I've seen are 320-330'ish...
Aim for a M110...your battery and motor will thank you in the long run. That might put you into the 380-mid 390 range.
I'm 50/50 on the yellow CA piston...I've seen several that have had tons of use and still look brand new...others crack/chip/fail with little use. Hit and miss I guess.
Definitely reshim, regrease, check that your nozzle is sealing well, check the piston head o-ring. Other than that, you're good to go.
Note: If it's a "well-loved" older rifle...it's best to go over it carefully to catch any weird issues.