Sounds a bit too powerful. The active hardeners already give a passive bonus of (i believe) 25.

They really use hardly any accumulator to run.  If you dont like how much they use, I'd suggest just waiting a while and it will get much easier.

compare mass and fitting costs of the universal (passive) hardener, and the active ones.

not when u consider the fact that they will never be as good as active hardeners

and i think they are just fine:
Chart

see, a single active thermal hardener negates the natural gap in resist of the blue mech.
while a single universal plate may have more effect on the resists, it has 2,5x the mass and almost twice the CPU costs of the aktive ones.
Ontop of that - Universal plates are MUCH more expensive

one simple question arises - what do you want to archieve with this change? where is the imbalance? where is the module that doesn't get used by any1, or the module that is so overpowered that every one prefers it above the alternatives?

a universal plate will eventually give me more overall resists, but it doesnt help me at all against my greates weakness as much as an active hardener. i have to stack in 3 uni hardeners to fix that hole with the same efficiency as a single hardener does, that
- takes a really small ammount of AP,
- adds by fare less mass to my bot/mech
- doesn't consume all those CPU points that i need for offensive modules
- ....