I've done a taccom build, a jp, and an MPX. They will all get you to GM. I personally prefer the MPX and have since sold the other two. Now you also have options of a taccom MPX front end. The softer impulse is nice, but what I like most about the MPX is the mags. They are 100% reliable period. I haven't had a single malfunction on my MPX and have a tax stamp in to make it an 8 inch SBR at some point hopefully before next season.
All you need for the MPX is a hiperfire trigger, a stock adapter, a buffer tube, and your favorite AR stock.
The only reason I would go back to a blow-back is to tune one to easily bump fire like Max Leograndis. But my .11 splits aren't going to be the thing holding me back for a long time. My gains are in transitions and footwork at this point. Master at the moment and first two GM classifiers next season will get me to GM.