If it is a short technical track like you stated then torque is more important than HP. I have had almost every type of engine out there and almost every price range and I can tell you from experience that going with the least expensive is better if you are new to nitro. Just like anything else there is a learning curve and it is better to make a mistake or two on a $200 engine then it is on a $400 engine.
For the RC8 you should consider the Novarossi N21B I had this engine and it was great. Here is a link to take a look at,
http://www.amainhobbies.com/product_...ducts_id/31073
For the MBX5T you should look at this one as a possibility,
http://www.amainhobbies.com/product_...ducts_id/29253
Hope this helps.