There is a database of set up sheets which cover different surfaces and configurations, look here:
http://www.petitrc.com/setup/losi/Se...TwentyTwo.html
Personally only ran rear config on grass and very wet astro, set up was pretty much the same as mid car with 80g weight as a block under rear shock tower/bulkead rather than under lipos... plus tyres were more suitable to conditions... didn't run weight up front as full stick lipo does that job.
There is a difference in the way the 2 configs drive, you can be a lot more aggressive on tighter sections with rear motor car and have to adjust to driving jumps with throttle as they can't be controlled in air as easily as a mid car.
HTH
Mark.