I think there must be a quicker way, but I admit I can't exactly find one. 14 to 15 hours to get from Porto to Burgos, and then have to backtrack the 200 km from Burgos to Leon seems like torture. It's entirely possible that this Porto to Burgos bus will stop in Leon, since it seems to be right on the main route, but it would still be a long trip.
Another thing to investigate would be Porto to Vigo, and then Vigo to Leon. Vigo to Leon is about 5 hours on the bus. I have ridden the bus from Lisbon to Santiago, and I know that it stops in both Porto and Vigo. Those two cities are less than 200 kms apart, so it can't be that long of a trip. I believe there is some regulation that prevents you from getting on and off in the same country if you're on an international bus, but that wouldn't affect this trip.
I've found that it's very hard to do international bus searches, so I'd be interested in knowing how you found the Porto to Burgos information, falcon.
buen camino, Laurie