Well, I'd expect a 20mm conventional autocannon to butcher the front armour of an M113, so I don't have a problem with a 25mm HEAP-firing automatic weapon butchering the front armour of a futuristic battlefield taxi, really. If I did perceive a problem I would rather solve it by upping foes than nerfing players, as the second is horribly demotivating.
Lascannon probably needs more PEN, though. It should be the tank-killer of choice.
. Part of the problem with Plasma is this one too: so you add +1d10 to all Plasma and it still sucks as we can see above. What do you do with the advanced Tau Pulse tech then? Give it also +1d10? That would make Fire Warriors extremely deadly to Astartes. I think we can agree that Plasma needs a boost but that will in turn require adaptation of melta and Tau weapons. In fact if you adjust Tau Pulse rifles you have to look at the other Tau weapons. It all ripples through the entire system.
Anyway, do not forget that Boltguns can cause glancing hits against Land Speeders or Rhino/Predator/etc. rear armour in the TT. So I am not entirely convinced by that approach. The Boltgun and the Heavy Boltgun should hold the capacity to do some damage. The HB's ROF needs to be limited, I suggest to 6. And if you look at my above calculations and figure in the proposed damage reduction of 2 to 3 points for Bolt weapons the numbers don't look all that bad.