If you have a E-Firestorm then I would get a stock 27t motor. The co27 and the P2K2 are IMO the best. But the co27 has a slight advataqge in power. In the stock class it will be for beginners. There is also a super stock class which allows 19t or 13.5 BL.

BTW you can race a 17.5 brushless motor in stock.