USF has the best scaling mainlane infantry in the whole game. 2 upgraded vet 3 Rifleman outperform 1 vet 5 Volks+1 vet5 Ober slighty. The infantry matchup isnt the problem. The Calliope nerf is justified and reasonable. Scott and Pack Howie are still decent, but require more micro in a micro heavy faction.
But i agree that stock rocket arty should be available for every faction. Its just too important. Dealing with anti tank gun/mg walls without arty is really annoying and way too punishing. Tbh if you nerf Calliope a little bit more you could just swap it with scott. Stock Calliope would make USF way better in teamgames and way less dependent on 1 or 2 commanders. UKF (and OKW with BGHQ, but atleast there you can backtech) lack rocket arty too and thats why they struggle in teamgames.
I don't think anyone ever complained about USF/UKF infantry. Besides, infantry engagements are not really common once T3/T4 starts rolling out as you'll have different indirects, tanks, arty, brummbar or whatnot to actually deal with infantry. Scott is definitely a reworked vehicle now. The autofire is all but useless since it's low range but the barrage is accurate to displace an MG or something. Not really something you pick in a teamgame but still, reworked to a new role. Not really worth fuel and pop but in 1v1 I can see it being useful.
Pak is downright useless until vet2 for the HE barrage. There was a game where my 2 pak howis survived late game to vet 3 with 3 kills among them. One pak howi killed one and the other killed two models.
About giving USF calliope stock... you would either have to severely nerf the offensive stats or make it a one shot tank and keep the good offensive stats, and reduce the price accordingly... or just rework the calliope to a slow AF arty which can survive a bit better and is in a doctrine, late and more expensive.