I think the warhound is a fine addition with a leftover point, but it is not needed after a windrush. (she is def 18 out in the open then, unless fog of war is also up, which brings her to 20... (base def is 16 I believe)
Against ranged only. Without the dog, she's an 18 in melee, which is still on the outer limits of hittable.
As for the dog keeping up with Sorscha, I find it rarely has problems. Sorscha only moves 12 inches every round (15, if she charges), so it's not hard for the
SPD 7 dog to keep up via running.
Three jacks is probably Sorscha's limit, although she's perfectly capable of running that many (especially at this point level). Barring Beast-09, the Destroyer is probably the best choice due to her feat, although the Decimator works quite well for taking down slightly harder targets.