Supposedly the KJW or WE Open bolt are the ones you want if you want to play out of the box and not really have to mess with anything. Other than putting in an "NPAS" kit to adjust your velocity so you're within field limits.
Note that FPS can vary widely depending on ambient temps (probably worse in Cali or other southern-ish states than for us Canucks where it's slightly cooler) and since it's a longer barrel there's more time for the gas to expand and transfer it's energy from expansion into the BB. Apparently some rifles have shot hotter than 450 FPS stock OOTB with propane.
gasguns.info is the site you want to go to for all the latest news and extensive knowledgebase they have.
If you ask me, I would probably say it might be worth it to look into getting some classic airsoft and going with a Daytonagun from AirsoftExtreme if you're in Cali and don't mind using a remote line and CO2/HPA tank. Too bad they only offer AK's (I'm assuming you're looking for a M4 GBBR). Otherwise look at the bigger picture and make a plan on what you want to get out of your GBBR system, whether it's a kickass sound/recoil and you'll hav fun with but also keep your AEG in the car as a backup, reliability and less blowback force, etc.