Hmmm ... maybe. I think if it was as strong as the full internal protection then a ship with 3 x MRP would effectively be immune to heat.
I suppose one solution might be to make them protect against heat as they protect externals (half protection, but cumulatively that means a lot less than half the value of stacked MRP protection of internals).
This is a nice idea.
I suppose really Frontier ought to permit general modification of MRP's as with HRP's. Lightweight would be popular. Admittedly it doesn't have many downsides, though.
Man, you're falling for it. Fdev have got you crawling down the rabbit hole for them at this rate. Come back! Turn around, come to the light! Engineers are NOT the answer!
I would rather that Heat Sinks simply not be ammo-based, and instead work on a recharge; whether that should mean they be moved from utility slots to a dedicated internal slot and only 1 be allowed at at time, like Shield Generators, I'm not certain on. But it would be the same thing I'd do to SCBs, if I could. Anyway, this would address heat-management in a satisfactory way and would mean we could leave MRPs alone.
Also, speaking from Mechwarrior experience, ANYTHING that grants you effective immunity to heat, in a game where heat mechanics are present and a key aspect of combat management, is overpowered as ALL HELL and should be abolished with gusto.