I tether my mini to my iPhone all the time. Works fine. Never noticed a difference in speed between the tethered ipad and my iPhone natively. Don't have a cellular ipad to compare, but it tethers well.
Hazy, any problems with upload speeds? I got a Verizon LTE iPad that had horrible upload speed much worse than my Verizon iPhone 5 in the same place. I thought it was an aberration, so I returned it for another and had the same problem