With the speed of an m8 and the fire power of an m4/76 imagine four of these with an m3 running a blitz attack on a town. This could in fact be a game changer
Not when it takes 3 rounds of HE to take down a building.

The US 76mm M1A1 cannon has the weakest HE tank round in the game. I'll be surprised if HTC even gives the M18 the option to take a full load of HE, TD units usually did not carry HE rounds on par with the AP ammo (they carried some, but certainly not comparable to tanks). I also would not be surprised if HTC gives some of the high velocity AP rounds to the M18 that were available to the76mm M1A1 cannon.
Its main purpose was to hunt tanks, not tear town enemy defenses and engage infantry.