Atm I'd agree with you on the Bushwacker B, it's nice on paper but it's biggest problem, ever since the armorincrease is ammo.
Under combatconditions, ie missing a few rounds two Bushwacker Bs dueling will run out of ammo before either is dead, it ends with them plinking away at each other with their SPLs which is quite pathetic.
However keep in mind that an internal damagesystem is in the works, which will ultimately result in close range weaponry getting "critseeking" abilities which might boost the usefullness of variants like this.
Scat E: never use, feels like a "frankenmech" to me...
I think the Demo is just fine, that thing is supposed to be a real threat to mechs, and srsly a Thor does have rather low armor.
The trick with tanks is to blind their view, which is rather easy due to their low speed. If you use stuff like RACs, low caliber ACs along with SRMs or even mguns (just for the particleeffect) tanks suddenly start to miss a lot of their shots because the have trouble to see you.
example Bushy A, RAC5 doe a nice job at blinding the tankdriver, and while they cool down the mguns work reasonably for that as well, add the screenshake of the SRMs and that Demo is going to have a hard time hitting you.
So much for the theory...

oh and keep moving, turret turnspeed of tanks is rather low, so you might even outrun the turret if you circlestrafe a tank, thus minimizing tha attackwindow for the tank.