I have to agree with mfr02, this is going to be a balancing act. To get light weight and fast is possible but at the expense of run time. What you really need to to think about is which of the following do you want to have:
1) a top fuel dragster
2) and indy/F-1 style racecar
3) a NASCAR style racecar
A lightweight boat with a small pack will give you what amounts to a dragster. It will be fast but only last a lap, two at best
An indy car is heavier than a dragster but will run further. By adding a second battery pack, in parallel, you will be able to run further, but not as fast, The added weight will lower your top speed but give you roughly twice the run time, three to four laps
A NASCAR is the heaviest of the examples I listed, but also the longest running. By sacrificing a bit more weight in bigger packs and maybe better motors and ESCs(yes, I know, funds are limited), you would be able to run more laps and still have respectable speed.
What it comes down to is do you want to dazzle everyone for a lap or two, be faster than the rest for three or four laps or be able to run with everyone for a full heat?