No it doesn’t sound reasonable at all. Sounds like BS to me. They were balancing wheels at NASCAR tracks with bubble balancers for years and driving well over 150 mph. A high quality tire should not be out of round and any properly set up balancer operated correctly should result in a satisfactory ride even in excess of 100 mph. When you returned with the problem did the mechanic put the car back on the lift and remove the tires and re-balance them? He should have.