In stock spec racing, unless your a seasoned driver, you wont notice a difference. I dont notice a difference between my 3200 20C packs and my 5000 40C packs in my stock slash. I can run 2 races with my 5000 though

For an 8 min main, I would look into getting at least a 3500mAh battery. I would honestly just get a 5000mAh pack. Its easier and cheaper to buy the best now then to have to upgrade and buy again later.

Some tips to keep you safe while using LiPos:
1. ALWAYS charge with a LiPo charger, in a fireproof location, at the maximum rate suggested by the manufacturer.
2. Use a balancer while charging. This keeps any of the cells from being over charged, which can result in a fire.
3. Never discharge a LiPo battery past the manufacturers recommended voltage. They are not like Nickle batteries, and become very unstable when discharged too low. If you notice your vehicle slowing down, stop it immediately. Do not try and "limp" it back, stop it and unplug the battery.
4. If you think your doing something wrong, just ask someone.