A friend has his complete 105 5800 group that I pulled off his bike in an ultegra swap still sitting in a box. I was just thinking tonight about what he could do with it when I had an idea...I have a diverge with sora and mechanical discs.
Would his shifters properly operate my mechanical discs, therefor allowing me to upgrade from sora to 105? In my head, it seems like this should work...

Sora is 9 speed, no? I’m assuming different pull ratios, so no.

As far as the brakes go yes. Any mechanical road disk brake should be compatible with them. Mountain bike mechanical brakes use a different pull ratio.

Thanks!