Fuck it I'll make a new post, sorry moderators:
Okay, so, to make a general statement, the KT and the JP4 perform better than the IS-2 and SU-85.
However, the OKW vehicles also cost more.
When a JP4 fires at an IS2 from max range, it has a penetration chance of 170. IS-2 has armor of 375/205 (front/rear)
So, penetration calculations use the simple equation p(%)=v/a
Where p is the probability of a successful penetration (expressed as a percent), v is the penetration value at a given range, and where a is the armor value of the tank.
Against the front, the JP4 at maximum range has a penetration chance of .453(repeating 3). So you've got a 45% chance to penetrate. That's far from "always".
Let's look at the SU85 vs. KT.
When an SU85 fires at a KT from max range, it has a penetration chance of 180. KT has an armor value of 375/225 (front/rear). When you take the values (180/375) you get an exact .48
That's not much of a difference, three percent, but it does better than the JP4. Against rear armor of the IS2, JP4 has a .83 chance. SU85 vs. rear armor of KT is a .8 chance.
here's a fun google doc I made in ten minutes.
https://docs.google.com/document/d/1g1GN0VO18kDoERcvo7q_4aKuyQpnN5dM5Jra0RflORE/edit?usp=sharing
stats from coh2-stats.com/vehicles
Edit: To conclude, l2p.
"it's never a good idea to tech t4 against late game OKW" yeah, of course, you don't need the Katyusha. T34/76s can put you ahead too. Say you kill the first medium tank from OKW right after it comes out and you know they've got to wait on their command panther or KT or whatever shit to throw at you. You get map control back cuz your 2 SU76s and your vet 3 ZSU are all still alive. You tech t4, push out 2 t34/76s and end the game.
idk, I'm not too good with theory crafting sometimes. Late game is hard with the variables.
Katyusha, though, always good.
Edit #2: Realized stats from coh2-stats are from June 2015. Checked penetration, armor values, and costs on
http://www.stat.coh2.hu/index.php and everything was correct except for KT's fuel cost (up to 310). The pen. values that they list are the mid-range penetration values only, whereas coh2-stats listed near/mid/far.