Unloaded rpm isn't that much high than max loaded rpm. Id say there is usally less than a 5,000 rpm difference based on my eagle tree data. Motor kv will only be about 100 rpm less at the most for castle motors. If you do see a big difference unloaded vs. loaded, it's because your batteries are insufficient.
Check out my custom converted 8ight:
Sub 7lb, lowest CG of any 1/8 buggy