Electric Cooktop 30A-40A Breaker

Hi, I had a 10 year old cooktop that was professionally replaced. The technician indicated that the existing circuit was only 30A and for both the old and new unit it was under rated and it should be on a 40A breaker. I had a licensed electrician come out to take a look and he agreed the cooktop should be on a 40A breaker. The circuit is wired with #10 cable and the line run is approx 20 feet. The electrician said that because the run was so short, the #10 did not have to be replaced with #8 and there is no harm in running the appliance on the 40A breaker with #10. Is this correct? I've always heard you must use #8 with a 40A breaker. Any help/guidance is appreciated. Thanks!