A .21 in a stadium truck is nothing but a waste of money, 90% of tracks wont let you run them. I suppose it would be cool just to zip around in parking lots and such. A good .12 is more than enough on a track, like the RB V12 or OS tz, plus the run time, a .12 will run longer on 1 tank vs the .21.
A 75 cc tank plus the .21 is about 5-6 min vs the .12 and a 75 cc tank is 7-8 min. Then the added wear and tear on diffs, cvds spurs ect will add up in a hurry.