It will be very difficult to balance your fan... The first obstacle you will hit (like a rock, or other solid object)will throw out of balance your fan...
If not designed to run at such high speeds, you will shorten the life of it, and may well have vibration problems. Think about the vibrations going from the blower to the tractor! Not a joy for your internal gears and bearings/sleeves. With 50 PTO HP, you should be ok with a 86" blower under normal circumstances (ie: not a hard packed or wet snow, and under 16" thick), at a good moving speed of course. Remember that the lower RPMs will give you the best amount of torque available. But when dealing with snow, what prevails is how efficiently that snow gets pick up, and then blown out of the shute. Higher RPMs are way better at acheiving this.