Yank doctrine wasnt to "out-tank tanks with tanks".
It was to outmanuever enemy tank formations and then hit them with combined arms, and yes, very much so, with TDs.
So it could be argued that Yank dochtrine isnt accuratly represented in the game due to the lack of TDs, "tho the firefly can be used in this role since its fast enough".. Herehttp://www.geewiz.net/history/634hist.htm is an interesting story on how the very fast TDs were used after the break out into the North Central plains.
We need a Yank TD. I believe. 
Actually thats how I use M8's
Who's largest asset is its speed.
Zip out to where the tanks are. Ambush em, or ride past em and come up behind them. Knock out the turret, then the engine.
Then leave em to whither on the vine while I go look for another.
Or roll past them firing at them to draw their attention and fire away from their objective or those in heavier armor.
You can slow to a crawl an enemy advance this way and buy time for those in heavier armor to get to town or the spawn area and set up a defense.
I find with practice. In reasonably clear terrain you can actually become pretty good at gunning on the run
If introduced, the M18 would become my main GV ride