Never seen a drag car with bags and yes they will vibrate off. 1 1/4 mile pass every now and then probably won't do it, but when you actually ride your bike for miles on end it can happen. Seen many desert cars lose their bodies do to it.
I would put my $75 job up against any other out there. It only matters that your welder is good. If you want to pay a crap load of money go ahead. Who's to say that the guy you are paying big money to is any good. No bondo cracking on my neck and rides straight as an arrow.
I guess it feels like it is the whole bike. It will happen when in a turn and hit a bump. I ease off the throttle let everything chill out and then right back into it. It can happen when nobody is around me going straight and I get on the throttle to quick, but could possibly be caused by wind...
When I ride 2 up with the TP I have no problems. When I ride single with TP on and get up into the 90s I get a little wobble. Need to check my pressures first though before I speculate and then ride solo again..