1) Blood is not better single target -
see dataz here. Also Unholy have superior unique raid buff, no one else can increase disease damage, no one else can increase aoe magical damage by 13%. Same goes with aoe and I am still wondering why the hell Blizzard nerfed HS aoe.
I think the conception of "blood is best for single target" comes from patch 3.1 and Ulduar times - then it was true. Since 3.2 and especially since 3.3 it is not true at all.
2) Going for 2xT10 ilvl251 from 4x T9 ilvl245 is sidegrade single target and dps loss on aoe. Chill and save badges until you get 300+ for four pieces. Also remember to respec to reaping spec afterwards.
3) Expertise cap is - 6.50% - 26 expertise - 214 expertise rating - before talents and racials. Pre-cap this stat is after pre-cap hit, STR, ArP, haste and crit, post-cap its useless - you can see exact values on first post.