G&P mechboxes are excellent starting places for upgrades.
BUT, before you take a shot with it get an experienced guntech to open it up and shim everything properly.
G&P gets pretty awesome air seal... but if you're so anal about it and want it to be PERFECT get an modify cylinder set
9/10 that I've installed have perfect aireasl even with the nozzle on the cylinder head.
Systema parts are out dated in my opinion, they've started to move away from the general airsoft market and focus more on the military/law enforcement training market.
Right now Prometheus is top dog... but the price also reflects that, Modify is a close second