plasma is nothing but ionized gas, it doesnt say anything about its temperature - more then 99% of visible matter is in a plasmid state. the upper layer of our atmosphere is plasma, better be happy its not fire
I've heard this mentioned in-game many times. Vehicle weapons don't chew through buildings very quickly. Seismics beat vehicles by a wide margin. You could simply match seismics in building DPS and call it a day. Oh and no damage over time, that's just frustrating for engies.
Make plasmas heat much smaller, but area of effect larger so it becomes a niche weapon to cook lots of infantry slowly (say 10 damage per shot) or spam confined areas. Then it will be seen more as easy add on weapon teams get later on to attack last lolforts or commit to the tech in certain specific maps like slaughtered, money or rainbow road.