That hybrid isn't described in the above guide, but it is very viable IMO. Lately it has been more like 50-50 though. I actually play 2/18/21 hybrid a lot, was playing that about 1/3 of the time and full engineering the other 2/3.
I would try at least full MM and full Engineering in PvP and see if you like either. Snipers have at least 4 specs that are all good at different things at different times. Some classes have one spec that is OK, and the rest suck. Some classes have one spec that is viable and other specs that are crappy. Some classes have one spec that is by far the best, and some others that are good or OK.