Last year me and my friend did the Ladakh trip, me on p220 and him on his R15. And i envied him on the curves. And it was really difficult to keep up.
Till manali you don't need to bother. The roads are pretty decent. But the wrists and shoulders ARE going to hurt. My friend is a super fit air force pilot and he was almost cringing in pain at the end of the ride. But when you have got a knee out and a good road it almost makes up for everything else.

After manali, there are really bad stretches and really good ones too. My bet is you will be in pain but will not think about it too much coz your brain will be busy processing the scenery. Trust me on this one.

The only hitch is the time you are going. Am sure there will be snow on the road at that time at some of the places, and i have no idea how good are R15's otherwise brilliant tyres in slushy condition.

But my advise go on R15, you will enjoy the curves and the fuel injection will keep your worries to a minimum.

have fun.
am coming in august though.