Competitive? Sure! Just a whole lot of work... Anything under a 13 turn brushed motor takes a lot of maintenance.. You will need to cut it every 3 or 4 runs.. if you have a lathe and everything.. then sure.. go for it!
I used to run a 12x2 as my fav brushed for 4wd... If the D5 & D6 use the P94 series brush and you can get a hold of the XXX LeMans brushes (retailed for around $15 a set) then you might not have to cut the comm so often.. maybe every 5 or 6 runs. I used to be able to get through a whole motor on just 2 sets of these running every weekend.
Honestly though.. having had a brushless in my 4wd for the last several months... it's way easier... I run a 7.5 and most folks at my local track like a 6.5... It makes life a whole lot easier! That and LiPos.. just charge and go.. and go.. back to back