I think a lot of us tend to over-estimate the importance of critical hits and structure damage. Certainly, the Mk IV dies a bit faster than most once its guts are showing. Thing is... if a mech's internal structure is showing, it's already mostly dead.
A 3050 model T-wolf (or any other optimized mech with full armor) has essentially 2/3 of its health bar in armor and 1/3 of its health bar in internal structure. Now, sure, there are some nuances on top of that, since each individual location has its own health pool and losing a torso means losing an arm. However, these are heavy mechs we're talking about, and neither one is packing concentrated enough firepower to outright amputate any locations, so the damage is going to be spread out enough and stochastic enough that we can basically talk about these mechs having unified hit point pools.
A 3145 model T-wolf, shiny and new from your local loanfox dealer, has a hit point pool that's 2/15s larger than the 3050 model (pessimistically assuming that the enemy is shooting at it with weapons that receive minimum damage reduction). What's more, that hit point pool is larger exclusively as a result of increased armor. The portion of its hit point pool where it can face-tank shots and not worry about limbs falling off is 20% larger, which is to say that it's equivalent to a 90 tonner. On top of that, it has equivalent firepower and speed to the 3050 model.
What is pays is that the 3145 model T-wolf cannot tolerate the loss of a side torso, and if the armor gets flayed open and the internal structure gets chewed on for a while, there's a chance that it dies from engine crits. But here's the thing; by the time that happens, the mk IV has already lost 70% of its hitpoints. So the mk IV dies a little faster when it's mostly dead already, but it takes 20% more punishment to get to that point.
That's a straight upgrade. The ferro-lamellar more than cancels out the XXL engine, at least from a survivability standpoint. The heat problems from mounting jump jets are more of a concern, as is the fact that you need to re-finance your entire planet to afford the silly thing.