In general, you lose some protection, as a two-stage exploit might write some data in W state then flip to (or wait for) X state for execution. That being said,

In general, you lose some protection, as a two-stage exploit might write some data in W state then flip to (or wait for) X state for execution. That being said, a self-decompressing binary would only do this at startup, before it consumes untrusted input, so given a way to drop map-executable permissions that wouldn't be a problem.