Not with a SFE. If you want a 4/6 SFE tank, you may as well go 70 tons instead of 75 tons, as you'd actually gain a spare half-ton. And compared to a 65-ton tank, you'd only lose a half-ton going for going 10 tons lighter.
I usually find 4/6 a bit of a wash for tracked vehicles, between all the motive crits they're easily going to sustain early to knock them to 3/5 or worse (usually worse), assuming they're not knocked out by a lucky TAC first.
I agree 3/5 is not useful, bare-minimum if that, but if you want a truly mobile platform and you pick a heavy tracked vehicle, you're doing it wrong.
Generally if you want "Truly mobile" you grab a battlemech, or a VTOL, or a Hovercraft.
I'm talking "enough movement to be marginally useful"-which is 4/6 for a track, 5/8 for wheeled, or 6/9 for Hover based on terrain penalties and the idea that you might actually want to get at least one good shot in before you're disabled against an equivalent opponent (i.e. an opponent that can administer at least one ten-point hit at ten hexes or more.)
a vehicle STARTING at 3/5 might as well be a fixed installation, (in which case you can save a lot of money by just building turrets and trenches). this is because it's highly unlikely to actually REACH a good position before the good positions are already bypassed or overrun unless you start it there.
ymmv and I built up my own 'doctrine' back in the days when I was the weird kid who took conventionals instead of 'mechs up against 'mech players-before Total Warfare added the extra die-rolling and adopted Maxtech's ideas.
but it still fits. outside of "Exterminate everything on the map" scenarios, positioning is important-being able to get your firepower INTO a good position is still more valuable, more often, than the raw amount of that firepower-iow an alacorn that can't get line-of-sight on target is worthless, even if it's otherwise (Aside from motive hits) pristine in all its armored glory.
Firepower is still important, mind you, but a shot you can't take, or that will miss, is worth zero damage, while a smaller, lesser shot that you CAN take, and MAY hit, is valuable, because it does damage, while the missed shot does not.
The problem with 3/5 is that more often than not, you will miss more shots going out, than your opponent will, and the only way to counter that is to simply put the vehicle in park and don't move-in which case, you don't have a lot of need for a fusion engine, or any engine.
in essence, 0/1, 2/3, and 3/5 are identical options to 0/0 movement. in all cases you need to START in a good position and stay there.
4/6 is the BEGINNING of "Okay, I have options to play aggressively or manuever my forces in a flexible defense." It's not "Optimum", it's "basic."
the major shift with Total Warfare, was to make slow vehicles viable-by placing a heavy emphasis on motive hits-but that viability is limited to "Start in a good parked position." They're still not viable for flexible tactics or on the offensive, because they will end up stranded with a locked turret more quickly, with less effort and less chance to actually administer that higher damage. They translate from "Marginal asset" to "Liability" very quickly. 4/6 is the slowest your tracked tank below 80 tons should be going, because it's the slowest speed at which your tracked vehicle is of any use as anything but a fixed installation.
