I wouldnt have mentioned the Su85 here but now that it has been discussed a lot already:
For me in taxing 2v2 games there never seems to be the right time to transition from Su76 to Su85 since the fuel investment is so great (90 T4 + 120 Su85) and the performance margin is rather small sub vet3. Then at some point your AT capabilities suffer, but still with this insight you couldn't do any better next time. The Stug for example is situated between 76 and 85 but more cost efficient and still in the cheaper tier of the two..