I own 2 Barefaced 210s but mainly only ever use one. I spoke to Alex originally as I wanted a big driver cab (my previous rig was a Ashdown MAG 210 combo with MAG 115 extension cab and preferred the 15).
However, he talked me into the 210s (they were a new model at the time with his talk of extra excursion in the drivers and the way he did his ports.
I was sceptical at first, but I have never experienced any issues with low Bs on any of my guitars at any volumes, and I can get pretty loud (occasionally). The pair of the cabs set up like a 410 is immense, but I rarely play gigs that warrant the need for a 2nd cab.
Having said that, I have no idea if it would fit in an MX5 boot, but I would hope so. My brother has a mk1, but it is buried in storage at the mo, so can't try it for you (plus I'm down south and he's near Newcastle).