Bottomline; if 25% penalty is too big, we could consider slightly smaller penalties.
As the strength of call-ins and their place in the meta varies considerably and applying a blanket nerf would be undeserved, I've gone through all (that I can recall) below.
From ingame experience / watching games, I'm fairly sure the
majority of call-in units do not need any penalty whatsoever.
The majority of call-in related balance issues have stemmed from singular abilities across a variety of the game's lifespan (currently it's M10, Command Panther and M4C). Adding further penalties to all would be to the detriment of game variety.
No penalty
Stug E - Not a problem unit in itself, and the Stug E - Tiger no tech strat is both unique and has not been problematic since the Stug E stopped being capable of countering light vehicles.
Command P4 - Not a problem unit. Unlocking it from tech would perhaps allow for interesting multiple light vehicle + command P4 strats, or Pak43 based builds since it has fuel cost now.
Elsewhere, you had better teched up to T3 (or T4) or be dead, because the CP's come too late to have a panic-P4 anyway.
SiphonX already answered this- no reason to further cripple a weak strategy if it's already so ineffective.
Puma - as SiphonX already said- not a problem unit. Players figured out how to counter Puma spamming with command P4 even before light vehicles in general were nerfed. It would be refreshing to see this strat again.
Mortar Halftrack - self explanatory- low impact support unit. I think this unit could use a price drop to 30F and CP drop (maybe even to 0CP) just to diversify openings, as it arrives too late to skip over the regular mortar and costs a not insignificant amount for a unit of the type.
Tiger - T2 to Tiger strats are not only completely inefficient outside of mech assault (you would certainly have enough fuel for one T3 unit by 13CP unless the game was already lost), but is also straight forward to counter. There's no need to make this playstyle even worse than it is.
T3 to Tiger strats, as mentioned in a previous post, are quite balanced as they are. Executing this with Lightning War is the strongest due to G43s, the osttruppen button and SCAS. With SCAS and osttruppen button being deservedly nerfed and T3 being rebalanced (stug nerf / P4 buff) there's no reason to further weaken this.
I'm sure that if you went T3 you can also afford 60MP/10FU.
So if it makes no difference to T3-Tiger play and cripples Stug E- Tiger play, might as well leave it out entirely.
Ace - How this unit fits into the wehr roster I'm not sure, after the change to full fuel and MP price. Holding out for the Ace without any tech has always been inefficient, so having the 25% penalty now seems completely unwarranted- a similar situation, in other words, to the regular Tiger.
I'm not sure why it needs the not insignificant penalty now (total cost tops off at 1000+MP, 400+F at half map control / full popcap).
Elefant - Improvements to utility and drop in cost might make this unit an occasional sight in 1v1. Fortified Armor with no tech might even be possible in troll games (command P4 to Elefant). Penalizing this unit for call-in without tech is unnecessary (it isn't a balance problem in 1v1) and would eliminate what might be an interesting option.
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M5 - Again, low-impact unit that's not actually seen in 1v1 play. With the changes to assault guards perhaps it'll be more viable, but that remains to be seen.
KV8 (current iteration) - I used this unit as an example in the original post- T3 + KV8 strats have been offmeta and easy to counter for ages. Removing the penalty would also allow for interesting build orders with Terror (T3-KV8-Howitzer). I agree that should the unit itself be further buffed then perhaps this could be re-examined.
IS2 - What SiphonX said- IS2 play is painfully rare, and nerfs to T3 will make stalling for this unit even harder. No reason to completely nullify T3 - IS2 play with the penalties.
I'm not convinced that a no-tech IS-2 can be countered at all with Stug nerfs. This is especially after the bruising you will get from SU-76 if you over-rely on T3 tanks.
If you lose the price penalty we (the playerbase) can test this change and let you know if it's too strong. Since you've improved the Panther for the express reason of allowing for heavy vehicle countering, I don't think this will be a problem.
ISU - Hard to say how effective this unit will be since there have been significant changes to how it works. Going by the previous iteration of this unit, playing around and countering the ISU is not very difficult, and again, similar to the IS2, T3 has been nerfed making stalling out harder. The unit is barely seen in 1v1s, similarly to the ISU.
KV2 - Doctrine basically requires T4 anyway for maximum efficiency. Should somebody want to attempt a KV8-KV2 centered strategy, I've no doubt that it would be both below the effectiveness of a normal tech-tier build based on the performance of the units, and very possible to counter. Requiring a tiers to remove the penalty would just hammer the nail in the coffin for this possible strategy.
Penalize call-ins without tech/tier (or lock fully behind tech)
Sherman M4C - This unit, together with the Command Panther and M10, is probably the reason for the vitriol centered around call-ins- as it actually lies in the centre of current meta balance problems. As it's most similar to the T3485 and Easy Eight, which themselves caused many balance issues back when they were call-ins, it's probably for the best to either lock this unit behind tier 4 (as with the other advanced medium tanks) or keep the penalty.
KV1 (with revamp buffs) - Before revamp buffs I'd argue that stalling for KV1s made for a strat that offered counterplay and was not overly strong. With the received damage and vet change keeping it behind tier / with penalty is probably for the best.