Lascannons are fine as the rules stand.
I think the only way you could make them 'dakkier' is to make vehicles harder to kill, whereby you could give the Lascannon a special rule that means its more effective than other weapons when it targets vehicles (more so than another high strength weapon anyway). This wouldn't stand within the current rules.
IF you really wanted to change them then you could say they count as
AP 1 against Vehicles when determining damage on a Penetrating hit (given the high explosive nature of the bolt once it penetrates its target) or something like that...but then you may as well make them
AP 1 anyway...
I still think they're fine - if you're unlucky, then you're unlucky...