well, as you know the more mah you have the higher avg voltage you will have over the 5 minutes. The ONLY reason to not run a 5k pack is the weight. Run time is not something you worry about in stock car racing. You do however want a higher C rating, over 50. Most of use run shorty packs with higher C rating. The Higher C rating relate to lower IR + more punch. Basically less voltage sag when you get on the power thus quicker. So a higher mah and higher C pack will have a higher avg voltage over the 5-10 minute race than a 2000 mah pack over the same time frame.

Here are 3 really good lipos for a great price. If they ever get them in stock. The Chinese new year has back ordered a lot of stuff.
http://www.promatchracing.com/prodde...prod=2S460080C
http://www.smc-racing.net/index.php?...product_id=127
http://www.smc-racing.net/index.php?...product_id=121