Is it really just the batteries that are in question? are we to assume that all other things are equal and you are as good a driver as everyone else?
I'm not having a pop at you but if the Club 500 race boat is an 'everything in the box' kit, including all the hardware, inclusive of motor coupling, shaft and prop, etc, then maybe it is the batteries. But is your hull built exactly the same as all the others, propshaft angle,prop balanced etc..etc. Have you run your standard 540 motor in or is it still a little too new?
Running higher rated cells (which are often that teeny bit heavier ) might not be your only answer, set up can make a difference, sharp corners on your moulded hull reduce drag giving you more speed. too much grease in prop shafts is a common problem it all saps energy too - Driving a better line might also give you better times round a course too, every time you use the rudder you're reducing speed.
Check those things first, then find your best performing pack out of the load that you have and see if they make a difference before you throw more money at it.