i never care about the xp penalty. i always ask people if they want a full run and if they want the bonus. same thing if in a HM i see someone undergeared: he has the right to see any drop the instance can provide. just talk..if you are short in time you shouldn't start a fp..
I do something the same way unless I'm feeling bad want it done is ... IF I see they are under geared I'll ask them ..
or other wise I let them know I will be skipping whatever I can .. and with groups were there geared I don't need to ask we just do it and skip all we can ..