On back to back trail centre testing over consecutive days, I’ve found bolt–through axles result in a 1.73% increase in tracking compared with QR axles. This is unfortunately offset by a 1.68% increase in weight, so it’s horses for courses.
I used the same bike, and the same front wheel with an axle convertor, and the forks were the same pair; I just swapped the lowers.
Having done the testing I’ve found both choices are compromised in some way, so I’ve taken to carrying both pairs of lowers and the axle kit on every ride, and I just swap them out depending on the grade of trail. I can do this while my riding mates are adjusting their saddle heights because I have a Reverb. I’ve found it best to attach the unused pair of Fox lowers to the outside of my Camelbak Lobo for quick access at the trailhead.
For Blue trails I’ve found QR to be more than fine. For Red trails I like the extra tracking performance and steering feedback of bolt-through. I don’t ride Black trails because my bike only has 140mm of rear suspension and I can’t really carry the spare frame I’d need to provide the required 160mm.
This said, to be honest even this current setup is a compromise. One of my favourite trail centre trails at my local trail centre has more left hand berms than right hand berms, so I’m hoping Fox are going to release a tapered axle fork with QR on one side and bolt-through on the other so it’s light where I need it and strong where I need it.