If you did want to bring the weight back up to the same as running nicd/nimh the simplest way is just to weigh your lipo and your nicd/nimh to work out the difference and cut a flat sheet of lead to the same weight and put it in the chassis under your lipo.
You may find with running brushed with a lipo that you get through brushes faster than with your old cells, but that's down to having the same power at the end of the race as at the start unlike the old days!
For racing I can't see you taking enough out of a fully charged lipo within 5 minutes to ever run it too flat, but if you wanted to be safe then Novak do a lipo cutoff that you can fit
Enjoy
