Hi. I've spent some time looking at this, and checking some of the other forums around the world through google trying to find a definitive answer. I was tempted to try and increase the rear travel to more than 140mm, by either having a shock worked on by TFTuned, or by using a combination of a longer shock and some offset shock bushings which could give an extra 4mm on a shock length.
Specialized state that their rear shock is 197x48 with 135mm travel (2.8125 leverage ratio). The BikeYoke conversion allows a 190x51 shock, which would give 143mm travel at that ratio (3mm more than the 5mm extra that BikeYoke suggest, but whatever happens it will should be at least 140mm if a couple of mm is lost). Some in forums have suggested that the're not getting 48mm, and others have suggested more. I hope that Specialized are being truthful in their assertion that it is 135mm travel, and am concerned by some of what I have read in this forum and elsewhere. And I'm putting my trust in BikeYoke when they say that their product will add a further 5mm.
I decided in the end that I didn't want to mess with the geometry too much, and to keep my upgrades simple. I also considered a coil shock, but with the progressive/regressive frame, decided to stick to air. So I've ordered a BikeYoke (SJ02), and a Rockshox Monarch Plus Debonair (190x51) for the rear. For the front, I'm increasing the travel from 150mm to 160mm by changing the solo air spring to a 160mm Debonair spring. I've also ordered a Charge2 damper to bring the fork up to a Pike standard.
If after making those upgrades the geometry needs tweaking, I may try some offset rear shock bushings. I'll post how I get on in a couple of weeks when I've fitted the parts.
Having said all that, what I should have done is bitten the bullet and paid the extra £1250 and just bought a Kenevo!