A houserule I was bandying around ages ago for DW was to double the Penetration of all weapons when firing at vehicles (the idea being that it made Penetration more important against armoured targets). Would that help at all?
I appreciate the argument that multiplication is a bit annoying in combat.
From what I can tell the new vehicle armour and SI values work great. Some tests I've done are avaible via the link in my signature plus I also tested the Leman Russ Battle Tank which also got some more reasonable TtK for all weapons. I'm currently learning for some important exams so I don't really want to devote to much time to doing another set of tests but I'll probably do one more once OW is released. The only problem that might still be there (besides some minor imbalances here and there) is Rate of Fire. I'm a bit concerned that the Autocannon might just be a little to tough in the hands of a good operator against some vehicles simply due to the amount of hits it could score per round, but so far that's only speculation.
I was always under the impression that the Autocannon was supposed to be a fairly decent anti-armour weapon anyway, although not so good as a Lascannon or Multi-Melta.