I went through a similar thought process. The XF shock is pretty good although always felt a bit over-damped to me. I spoke to Jake at Sprung Suspension and his view was the best in-line shock in terms of price/performance/reliability was an Ohlins.
I ended up buying my Mojo3 before so it never got done, but I’d take his advice, I’m sure you’d notice a difference but it might be hard to quantify that in terms of cash spent! What I’l probably do is spend £95 and get Jake to service/tune the XF McQueen forks. He reckons there is loads he can do with those that you’d really notice with the standard parts. The XF isn’t particularly ‘tunable’ apparently.
Anyway… Tom on the STW staff has a FlareMax with an CCDB in-line. He seems to rate it as an excellent upgrade…