I have a Gammill Vision 18-8 with a 12' frame and I quilted a 120" x 120" king with Hobbs 80/20 batting. It was a little bit difficult changing the bobbins because the backing reached almost to both sides, so I had to bend or squat to see what I was doing, but that really wasn't a big deal. If you are doing a pantograph on a quilt this size, you would need to use one with a narrower stitch path because as you roll the quilt onto the take up roller, more and more throat space will be taken. I was doing an freehand design so that quilt so it was not a problem for me.
I have done a couple quilts that required me to roll the quilt back and forth to complete a pattern due to not having a large enough throat, but, since it doesn't happen often, it's not big deal either.
My frame does not have a dead bar. I think it would be a nice feature, but it really is not a big deal to adjust as I can usually do 2, 3 or 4 rows between adjustments and those adjustments take less than a minute to do.
Good luck on your decision and I hope you enjoy it as much I do!!!