Sounds like an interesting variation. I assume you've ridden the Rannoch - Corrour - Ardverickie section previously? I'd say it's actually the highlight of the whole route.
I just couldn't figure out an easy way of linking the section from Killin to Fort Augustus, in winter/February, before almost all of the available B&B/hotels are open for the season. I was also trying to make sure that it was enjoyable for my boy, rather than something he'd never want to do again, so doing super-long days (we were riding for over 6 hrs each day as it was), or camping (-3º as we left Killin to head over to Glen Lyon) would probably have taken the fun away.
The alternative route across to Dalwhinnie was pretty scenic from Kinloch Rannoch, and the next morning heading down the side of Loch Ericht and then north from the lodge through Ardverikie was amazing, with stunning views across to Ben Alder, Geal Carn etc.
Distance-wise, it was about the same, possibly with slightly more climbing to climb 'up' to Dalwhinnie, then back through Ardverikie. 
Good on you @GavinB and your lad. I think I saw you both on the A9 section of the cylepath just about 4pm on Sunday 9 Feb as you were heading to Dalwhinnie. I was heading south in the car with my lad - we'd just been at Laggan - and I commented that you both looked like you were on an adventure.
@troutwrestler - that would have been us, for sure!
That cyclepath was FAST! Super-smooth tarmac, gentle gradient descending down to Dalwhinnie from the pass and then every now and then it was covered in ice and snow - perfect after a long ride up from Killin!
