No, Jamus, it's based upon extant rules materials. As I said in the post you misquoth, based upon MT rules. Which are close enough to physics for this discussion.
And since I decided to check myself, your idea that legs will be tougher must assume 6+ legs...
MT Ref's, page 85-86 states that the inop number si the same fration of the hul inop dp as the locomotion system is of the hull. this is then adjusted (Halved for wheels. x N/5, where N is number of legs for walker) by motive type for wheeled and legged. So, a bipedal mech has 2/5 the DP for locomotion of an otherwise identical tracklayer or grav vehicle.
So... what does this mean combat wise?
In addition to flyby effect (Getting hit by a faster unit doing a strafe before you can bring to bear on said unit), you have most mechs being less than half the toughness for the same volume of propulsion... a major weakness. "Bug Walkers" (6 or 8 leg), acutally are tougher per unit volume of transmission/suspension. (6/6 or 8/5 multiplier to damage capability...)
Now the problem is that the cool factor is for 2-leggers...
The intimidation factor of mecha is definitely a reason to build a few... but not, IMO, valid enough to make them Primary Combat Units. Police units, say, 3-4m tall, with machineguns, stickyfoam launchers, and autogl's, can be seen as a really effective crowd deterrent... small enuough to maneuver well, large enough to have good clearances, and heavy enough to stop almost all small arms... and protect the officer inside from even heavy arms to a point.
Police units have a different need-set from PCU's. Police mechs, for crowd control, at least, need to be able to outpace individuals, protect occupants, take down civilians, even armed ones, and operate in very cluttered close-in environments with little to no environmental damage hazard.
PCU's need heavy armaments, high maneuverability, high speed, and enough armor to achieve the mission, coupled with long enduances.
Envision, if you will, a 2m tall Bug-Walker, say 1x3x1.5m cockpit module, and about a 3m leg reach... outside, it normally walks with a width of about 3-4 m. Indoors, it can get down to probably 2m... it can pop-up the canopy, probably to about 4m high. It can probably walk faaster than a human run at TL11... and if you give it a very low thrust grav module, it can fly. (Limit the grav to just enough to lift, say .1G above local, and it doesn't even fear being lead off cliffs...
Need a reason for non-PCU mecha? look at the costs of gravitics in MT... a walker is FAR cheaper, and far less versatile.
combined modes, like the police bug above, are very valid uses of technlogy for limited purpose: realism says that since the parts themselves are more expensive, using the grav means more parts needed for it. and even that use, the grav might be as much as 1/3 the cost overall...
minimum cost per ton moved is 2000 for std grav, but note that the cost there incleds 40% of the mass being the grav system....
minimmum power:weight of 4 is 0.004MW/ton.. and that means a measly little drive system of Cr2.5 per ton-mass-of-vehicle + Cr150/Kl-of-vehicle. {(625*.004*tonnage)+(1500*.1*KL).}
Grav is expensive. Better (lower mass per ton of thrust) grav modules exponentially raise the cost.
The "Trubo-grav" concetp from Space opera is probably the best overall... gravitic lift, and maybe a little lateral thrust, plus turbofans (either IC or Electrothermal, or fusion exhaust infusion, or fusion collant heating) to provide high-speed reaction thrust cheaply. I've done similar in Striker...