It is possible the novel was talking about theoretical claims what could be done with TSM. Without any regard for actual engineering and other factors.
Evidently it isn't possible... but not because of TSM. TSM is basically stronger musculature for the 'Mech. But this doesn't mean the structure can take heavier load, and it doesn't mean the myomer muscle can support the structure (if it does that, it probably can't work as a motor at the same time, and in case such solution would be problematic because if power is lost, the supported structure would collapse).
If you can provide 'Mechs with sufficiently strong structure and perfect the TSM to not require heat activation (but also not cause spasming and jittering like industrial TSM), you could have 'Mechs that can carry more weapons and armor while retaining their existing mobility, or retain their current armor and armament with greater mobility.
Technically 'Mechs with TSM or supercharger or MASC are able to move faster with their current load but they only do so occasionally usually, and we don't know what kind of strain this cause to 'Mechs joints and structure. I seem to recall that TSM makes repairs more difficult...
EDIT A way around the structural limitations, if they truly are the issue as i think, could be in making double-weight and bulk Endo-Steel structure for standard weight BattleMech (eg a Locust would use 2 tons of Endo-Steel instead of 1 ton, at the cost of 28 slots). But this would be very bulky and material-intensive, and expensive. With this, one could possibly have a Locust with armor and armament and mass equivalent to a 30- or 40-ton (or whatever sounds reasonable) design with current 160-rated engine while retaining the existing mobility.
EDIT2 I'd also remind that PTSM rule is likely as it is to avoid being overpowered. Nominally, extra speed without any cost but weakness to one weapon type would be likely more than worth it. And at worst, it would devolve the game into "bring as much green smoke as possible". And then there'd be wondering as to why TSM isn't perfected and made standard for all 'Mechs...