One thing that would strike me about that change in BF/QS/AS terms is that, while the Structure points would go up to 3 4 (from 2, the amount an XXL engine in a 75-ton Clan 'Mech would equate to in that scale), the Element itself would go down to +1 to hit rather than the +2 it would enjoy otherwise.
In that scale, I would imagine the difference between "only" 2 and 3 4 Structure points to be relatively moot (given the danger which each critical hit roll would then present at that point); but the loss of the protection offered by the current version's move rate might make it harder to avoid taking so much damage as to risk hitting those Structure points in the first place.
Assuming I haven't misread the appropriate parts of StratOps, I guess. And given that I haven't actually played BF or QS myself, I'm sure that someone with some actual on-table experience with the game engine might have other ideas.
EDIT: I made a mistake with that one. In BF scale, a 75-ton Clan 'Mech with an XL Fusion engine gets 4 Structure points, but one with a large XL Fusion engine gets 3. Both standard and large XXL Fusions equate to 2 Structure points instead.