The draft FAQ wording suggests that the answer to your question is a clear "Yes":
Q: Do blast weapons, regardless of size or type, now hit all models at all height levels under the template? And if so, is intervening terrain (such as bridges or floors) ignored when determining how many hits are caused?
A: A blast marker or template affects all models underneath it, including those on different levels of a piece of scenery. (Designer's note: Earlier versions of this rule restricted the effect of blast markers to 'a single level' of scenery. This created problems with scenery that didn't have strictly definable 'levels' and we decided on this change for simplicity. In addition, we feel the rules now better reflect the explosions being three-dimensional (i.e., spherical and not circular) as well as better representing the deadliness of weapons such as flamers when used in confined spaces.)
Source: http://i.imgur.com/aFOc2ZT.jpg
So, if you shoot a Demolisher cannon on the top floor of a ruin, everything on all levels of that ruin are now hit, provided that they are at least partially beneath the template. Furthermore, as per the BRB on how blast weapons hit and wound, ALL of the models hit in this way can be removed as casualties, regardless of line of sight. BOOM!
