I've gone got me a slash to race with. If I go into a modified class what sort of motor/esc combo is the most popular and fastest?

The truck has velineon at the moment with 2S Lipo. I also just bought an ebay special that has Havoc 3s and Ballistic 8.5t installed. How much faster is this than the velineon, if at all?

Thanks for any advice. I'm going to enter my first race this weekend after being a track basher and nitro user for all my life but never racing so i'm keen to not get left behind!