Not sure what comes OEM in the UK, but the stock GasGas pipe on the US bikes is excellent. It has a great power spread and is heavy gauge like the FMF Gnarly. From what I hear it is similar to the DEP and Doma in performance.
FWIW I don't like the older FMFs. They killed the top end and in the case of the 300 made them yank real hard off the bottom. Plus, the GG Gnarlys were not made from 18 ga steel. The newer ones are a different pipe.
The Stealth, while a quiet silencer, is quite large. I prefer the smaller TC2 or just a silencer if no SA needed. With a good pack they are quiet enough. LV makes good stuff, I'll likely get one for my '12 to replace the Stealth.