Bikepacking - a viable alternative to racks & panniers

Looks like the perfect thread for this: me and a friend are climbing Snowdon the weekend after next in late evening, bivvi sleeping on top, then descending at sunrise. I've got an a Orange full-sus 29er and need to carry some gear. I was thinking backpack and Alpkit day bag on the bars. Sounds sensible? Would a 13L hold my bivvi with sleeping bag inside, and a small thermarest? And would that weight stop me having fun on the way down?

How big's your sleeping bag? I can fit mine (3 season, down) with bivvy and liner in an 8L alpkit bag. Reckon I could wrap the mat around it and slot it in the 13 with tarp and lines popped in on top.

Or just stuff it all in the 13L bag with room to spare