It's a 1 shot vet 1 ability, not a buff the to default fire. The m10 costs the same amount of fuel and can fire HVAP for 30 seconds, doesn't seem like a tall order
Per comments above, the Stug penetration bonus would last for up to 4 shots, which is plenty.
M10 is doctrinal costs 80FU which is dirt cheap, but it can bounce shots on vet 0 OST P4 and it fights targets that commonly run about 230+ armour. It's more of a cheap unreliable tank destroyer that also takes a commander slot. It's decent for its cost as medium repellent and good for suicidal attacks on enemy rocket arty.
In other words allies can pick a cheap doctrinal TD that can fight axis stock vehicles, while Wehr would like to build stock cheap TD that can fight allied doctrinal heavies.