You'll get an 8ight buggy well over 100mph on 6S using a 2650 with 43/25
I'd love to see a L8ight tweaked up for speed runs though. Can't imagine why you'd consider a 1521 though? unless you want to be running continuously...I'd use a 1515 1.5 - good for a couple of 6S passes before you let it cool down
Get some of these;
http://www.grpgandini.it/index.php?p...chk=1&Itemid=6
Vertical stabilisers & no wing at the back, big splitter at the front. Good to go