I have never liked any of the Bushmaster "BMAS" accessories. In every instance I've bought them, they haven't been of the quality or as functional as a similar product from another company. They tend to be heavier, chunkier, blockier, and usually, purple.
Specifically regarding the REAR sight, spend literally a few more dollars and get the ARMS #40.
BUT having said that, I must add that 1) I have absolutely NO direct experience with the BMAS Detachable Front Sight, and 2) it seems to be the exception to the rule... The DFS is very well liked here and elsewhere. Were I in the market for an inexpensive detachable front sight, I would give it a try.
The "lining up issue" IMHO is overstated (.040"???). If you can't get the rifle to ZERO, Bushmaster sells the taller front sight post which will fit in the DFS. Heck, just add it to your order with the DFS, and if you don't need it, sell or trade it in the Equipment Exchange forum.