Looks like SB Tactical is making a new collapsing brace for the Sig MCX rifles. From what I have read online, the collapsing MPX brace is not compatible with the MCX. If you look at the photo above, the adapter that attaches the sliding bars to the MCX receiver is different than on the MPX.
This MCX is in .300blk and Alex says it suppresses very nicely with sub sonic loads. This could be a contender to the old AAC and new Q Honey Badger.