All Lexus models are emission equipped for 50 States. Prices are up to the individual dealers and their own market situations. There won't be that many IS300's in circulation at first so the sticker price will probably hold which is the same everywhere. There may be pockets of availability that could get you the car more quickly, as long as your color and options are open ended. Try calling dealers around the end of the month to see what's left on the ground.
If you are a valid resident (e.g. own property) in some state other than CA then you may be entitled to buy a car there and register it there. Depending on sales tax and registration fees you could save money.
If you just go out of state and buy a car, then try to register it in California then they will still find out what you paid, charge you the sales tax, and hit you up for the same registration fees.
Used cars are a different story. California has had this weird $400 "environmental impact" fee on non CA cars (used) brought into California. There have been lawsuits about that fee and it is uncertain about the continued collection of that fee. They may even give a credit back to people charged the fee in the past.