3L engine?

I'm considering buying an LS-300... got a few questions. Is the 3L a good engine? How many horsepower can the engine handle without upgrading the internals? Anyone here have one with crazy high HP? What's out there as far as performance upgrades? Does the 3L DOHC come in any car other than the SL-300? What transmissions come behind them, and how much power can these transmissions handle?

Thanks
Jason