Array indexes, GIN?

I need to cross reference 2 tables. There are O(10M) A's, each has an 
ordered set of 10 of the O(100K) B's associated with it. The dominant 
query will be finding the A's and their count associated with a given 
list of ~1k B's i.e. if 2 of the listed B's are in A's set of 10, it's 
(A,2), and we should get back ~100K rows. The good news is we only need 
to run this brutal query every couple minutes, but the row updates will 
flow fast.

Luckily this is PostgreSQL, so the simple solution seems to be

   CREATE TABLE xref( A bigint, B bigint[10] ); -- A is primary key

which cuts down the table overhead. O(10M) rows w/array.

On the surface, looks like a job for GIN, but GIN seems undocumented, 
specifically mentions it doesn't support the deletes we'll have many of 
since it's designed for word searching apparently, the performance 
implications are undocumented. I searched, I read, and even IRC'd, and 
it seems like GIN is just not used much.

Is GIN right? Will this work at all? Will it run fast enough to function?