While it is true that you have to pay a fair a mount to get certain results it is still less than what you have to pay with a combat vehicle thanks to them needing an extra 50% engine mass in shielding to mount fusion engines and quadvees can use double heat sinks while combat vehicles can't.
So even counting the gyro it only takes 30.5 tons for a 100 ton QuadVee to achieve 5/8.
Compared to the 31 tons that a wheeled vehicle would have to spend, well it is only half a ton to be fair but it should also be pointed out that is only a 380 XL Fusion engine too.
31.5 Tons...Quadvees need a special cockpit.
The advantages of a QuadVee over a mech are that it can pay less attention to gyro and leg damage and can transform into a unit 1 level high. It can also mutitarget and has a spare cockpit/pilot. The disadvantage is that it gives up over20% of its entire mass to do so
Compared with a vehicle...the QuadVee can pay less attention to track damage. It can use certain BattleMech technologies that are otherwise forbidden from being use on vehicles. It doesn't have to pay for shielding. It can transform and enter terrain types normally forbidden to vehicles.
Comparing a QuadVee to a Mech - say the Ryoken vs the Cyrallos - the Ryoken has more armour, is more mobile, and carries more weaponry/equipment. Even if we switched the cyrallos to an XL engine, that would save only 7.5 tons.
Comparted with the Enyo - the Cyrallos is slower, carries the same armour spread over more locations. Even if we gave the Cyrallos an XL engine, again, that would only allow another 7.5 tons of equipment.
So - while you are right that the QuadVee pays less to acheive a certain movement rating than a vehicle, and doesn't have to pay for shielding, it DOES have to pay for conversion equipment and a secondary motive system that the vehicles does not. So while the Enyo has 6/9 movement from a 330XL engine, massing 19 tons including shielding, and controls massing 3 Tons , the Cyrallos uses a 275 engine (15.5 Tons) plus Conversion (5.5 Tons) Plus Tracks (5.5 Tons) plus Gyro (3 Tons) plus cockpit (4 Tons) to acheive worse performance.
22 Tons vs 29.5 Tons. Were we to use an XL on the Cyrallos, we would save 7.5 tons, making the two equal in mass, but with the QuadVee still a 5/8 unit instead of 6/9.
QuadVees are closer in performance to vehicles. Definitely. But still largely inferior. But the motive system and conversion equipment more than make up for the gain from the lack of shielding.
If the units were fluffed as vastly cheaper to build than Mechs, their lack of effectiveness could be excused. Or if they had some advantage over the other units...a LAM and Thorizer could both fly for example, which offers obvious advantages that could be worth the loss of effectiveness, even if it were simply used as a strategic mobility option.
But as it is...the QuadVees are interesting, but seem best viewed as a cheap flavour unit, an experiment, a unit with cheap BV - and one that probably won't be around long.
They are best seen and used as tougher vehicles - but even then, it would simply have been easier for CGL to remove some of the arbitrary restrictions on vehicle design.