Oh, you're talking about ACTUAL MDF enclosures, not these $75 FOAM boxes that LOOSELY fit around the speakers.
Yeah, I can see the benefits here. But these actual real MDF enclosures will cost A LOT more than $75.
The OP is talking about $250 in-wall speakers. So I doubt any of them will come with real engineered enclosures.
I was talking about these $75 typical loosely made foam enclosures:
But you are talking this:
This is the RBH SI-831R Reference in-wall used in Gene's new house, which is equivalent to the SV-831/R free standing cabinet of the $45K SVTRS towers that Gene has. Definitely not $250.