The Brummbar had a similar treatment as well. This along with a CP and cost increase should be good.
When and how did this happen? IIRC the Brummbär is still totally capable of wiping any number of models as long as they're bunched up closely enough or have low enough health - which both the ST and AVRE should also be able to do.
Now you can argue that the OHK radius of these heavy caliber shells may be a tad bit too large, but giving it the T-70 or Ostwind treatment would IMHO cause more problems than it would solve. The model kill limit may work well for units with high rate of fire that don't rely on alpha damage to be effective, but if a unit that fires one shell per minute doesn't even have the chance to nuke a squad on any occasion it will be rendered useless. On top of that, where would you even draw the line - 4 models? 5 models? or better 6? The effectiveness of the ST/AVRE would heavily depend on the squad size of the target you're facing, with low-model count units like (p)grens, pathfinders or 4-man sections likely drawing the short end of the stick while the already durable 7-man cons get a huge survivability boost. Not a good solution in my book and there are better ways to tune down the ST, like revisiting cost/timing, range, mobility (especially when reloading) or, if absolutely necessary, the AoE values.