As a recent owner of a M20K 231, with TIO360-GB engine, I learned that it would be very beneficial to install a Merlyn waste gate. So, this equipment is STC'ed by FAA but the manufacturer never requested for an EASA STC.
I could, and actually, without difficulties, file for a BASIC EASA STC that has been granted 2 weeks after filing and this STC is granted to my aircraft only. This is possible for all FAA STC's (for private aircraft and only when there is no EASA or former national STC) as long as the modification doesn't impact the noise produced by your aircraft, which is the case for the Merlyn waste gate. Cost of this operation is about 250€ per STC and for this waste gate there exist 2 FAA STC. So, you have to double it.
Anyway, I'm now a happy 231 rider, and confident that this investment will be beneficial to my engine (only 200h since MOH and installation of the Merlyn waste gate) and I think any GB engine owner should consider installing this item together with GAMI injectors.
Thought that this would be interesting to share.