Those BETD links were never a good idea for any of Specialized's older FSRs
But even worse for these old SXs as these frames have a silly high leverage ratio already and are prone to blowing through their travel far too easily so sticking on a linkage to give more travel from the same short stroke shock is just going to feel horendous.
I can't believe you've not experienced the drawbacks of such an overworked/underdamped small volume shock already, What do you weigh? I'm guessing you're either a really light or maybe you just don't ride anywhere with long (2min+) rough descents or rarely ride agressively.
Having your shock "tuned" is going to do next to nothing, there are only 4 shims controlling both rebound and compression damping inside your OEM vanilla. If you really want it to perform better the best money you can spend would be on a PUSH tune where your internals will be replaced for a far greater number of shims which can be tuned to your preference but even after that It will still lose performance from overheating when ridden down rough trails for extended periods of time.
One good thing about the super low BB height on these old SX frames is that they will still ride well if you chopper out the front with a longer (than intended) fork without leaving you with a sky high BB height and an unstable ride. the only downside of this is that the seat tube angle (which is already too slack) becomes almost un-useable if you are a taller rider so I'd go with a fork you can adjust back down on the fly to 100-120mm for extended climbing.
if you plan on fitting a 150mm fork, For reference: a 140mm Pike/Rev will give you around a 12.7mm BB height and 65deg head angle in the low setting (a degree steeper and slightly higher BB in the high setting you're using at present) so slacken off those angles about another half a degree again for a 150mm fork
One other point, knowing what these frames are capable of and how rough some of the trails out in Spain will be with Switchbacks your component choice (and overall bike weight) seems rather confused, stans Arch rims and lightweight tyres on an 8.5lb frame? seems like the older normal 01-05 enduro would be more the sort of bike you actually want, but if not, do yourself a favour and fit some stronger/stiffer rims (Flows if you really must keep the weight down) and a set of tougher tyres.
Jam bo? Out of interest, what sort of times were you getting down Ft William in on yours? I'm assuming it'd have been the old shorter track? (the one that started from further down off the walkers path to the cairn, you weren't meant to ride).