Ah yes, is that the rocky descent to the underpass? I remember thinking that a dropper seatpost would have paid for itself there, but that's another thread altogether.
Seriously though the WHW way has a bit of a weird character, in that it's essentially one long trail centre for ramblers. As such, no concessions have been designed into it for bikes. Uphill effort is often not rewarded with equal downhill speed/fun, which is where it quickly gets knackering. i.e every mile has to be earned.
As for the Devil's staircase chat, I've walked up and down it loads of times and couldn't imagine riding it, but I know guys who have. It's more about the rider than the parcours.
FWIW I'm looking at the Southern Upland Way but will be launching into a few investigative forays just to gauge its character. The guide book is next to useless, as it's from a walker's POV
Don't remember the under pass, I went out the station down the road and then straight on the whw, up the hill, took in the views(great btw) then downhill to the pub, pint of guiness before the next up and over to kinghouse, another pint of guiness, then up and over the devils staircase to kinlochleven for another pint of guiness 
I kind of agree about the reward thing, i'd heard loads about the descent from the top of the Devils Staircase to Kinlochleven, but i was pretty underwhelmed by it tbh. When i did it, it was terrible weather, raining, and i could hardly see 2 feet in front of me as we were in the clouds, so conditions weren't the best. But when you hit that fire road it's a bit of a disspointment, especially when you are walking up the devils staircase thinking; this would be great to come down.
Similar to the up from kinlochleven, thinking also that would be great to come down, though i did enjoy the bit after the up when you fly across through the peaks to the next fork at the choice between the road and the actual WHW into FW, i think that bit would be good either way. I took the actual WHW from there and that felt as if it would flow better the other way, plus there was a big long fireroad descent again into FW when i did it..actually more i think about it, N to S, aleast as far as orchy, does sound more rewarding i think. Hmm, I think i've just talked myself into that at some point this year!
