fast, mobile arty. I can see fast runs to take down shore batts. quick runs to town to start reduction before defenses can respond. running out to meet and bushwack GVs headed for town or field. much more useful the the 251.
Apple vs orange. A very big advantage the 251 had over the M3 was its ability to traverse rougher terrain and through snow and mud better (not an issue in AH obviously). Speed is not the only factor when comparing the 251 and the M3, even in AH.

Perhaps HTC will add more variants of the SdKfz 251 chassis?
251/9: 7.5cm infantry gun. Very similar to what we have on the LVT-4.
251/21: triple MG151/20. 20mm AA or infantry support.
251/22: 75mm Pak 40 (same as Panzer IV).