Allied tank destroyers already have the advantage of superior range and/or penetration over axis TDs, with the SU-85 already even having a comparable RA. The stug has a better RA as it necessarily needs to be more durable due to its closer proximity to enemy vehicles, and the JP has a better RA as its role is actually to be able to fight other TDs, rather than defeat heavy tanks (due to inferior penetration combined with higher fire-rate).
The SU-76 is the only one there I'd argue could do with a buff, but I still think the units' role should change, as opposed to improving its' target size. It's also a multi-role vehicle with its barrage currently.
The M10 is just a pointless unit in most cases.
In the case of the Jackson; It doesn't gain mobility with vet as it is ALREADY the fastest (stock) TD at vet 0, being beaten only slightly by the SU-85 and JP at vet-3 (and beaten quite convincingly by the Stug at vet3), while also having the best AT performance and having an usable turret. (and in the case of the Stug, a 10-range advantage). Despite the nerfs it has received, it is still an exceptional tank destroyer.
All in all, Allied Tank Destroyers aren't really in need of buffs.
according to price and timing. stug 3 must be compared to su-76. not 85 or jackson.
meanwhile axis no need to use TDs while panther are good enough.
currently Thing about tank phase that we should concern is popcap.