After running lots of brushed Mods and having experience with nearly all of the Novak motors. This is the best I can come up with (opinion). Remember that many brushed motors run differently but this is based on an average brushed.
SS4300/ 10.5 = 19 spec
SS5800/ 8.5 = 14-15t
7.5 = 13t
6.5 = 11t
5.5 = 9t
4.5 = 7t
3.5 = 5t?
I woldn't run less than 5.5 in any offroad racer.
If were picking one for offroad mod, I'd only run the 5.5 in 4wd buggy. The 6.5 is really nice in an ST and the 7.5 would be plenty and a little milder. The 7.5 or even 8.5 would be plenty for 2wd buggy, depending on track.