Maybe not exactly what you are looking for but I've been riding and racing my Trek Stache (setup 120mm) as my primary bike for over 3 yrs now. In some instances I can tell its not quite as fast (marathon races where you are transferring on the road/pavement). I've been in situations where I was essentially in a MTB peleton/paceline and I could tell I was not as efficient, but I think that was mainly the 29+ tire. But I put a nice lightweight wide carbon wheel set on it and I think it excels in the techy terrain. And I've done multiple 6-10 hr events on it with respectable results.
In summary, I say go for it! My fork does have a lockout so I'm not too worried about how much travel I have. When I need it...I can have ZERO travel.
All that said, I do think that the modern progressive STEEP STA would be a notable hindrance for me when pedaling very long distances that covered some flatter terrain. I have a hard time finding the right saddle fore/aft spot when the STA gets above 74* for me. YMMV
Cheers,
CJB
In summary, I say go for it! My fork does have a lockout so I'm not too worried about how much travel I have. When I need it...I can have ZERO travel.
Cheers,
CJB