Yes you are racking up more miles and going faster than the truck says. I think I'm about 15% off. As far as ride mine still rides great. Sure it handles a bit different being up in the air but the ride is still good. Drive shafts issues are hard to give a definite answer on. Some guys have vibrations stock, others have none with an 8". I currently have 2" of drop in my carrier bearing and I have no noticeable take off shudder but I do get some slight vibration on decel over 45-50 mph. I plan on adding a CV to the shaft to get rid of the vibration completely. Maybe others can chime in with what they've done to fix the driveline vibration after lifting because I'm certainly no expert.
Oh and you will definitely want to get a dual steering stabilizer whether you go 6" or 8".