I think it was Tonto and another couple of guys who did some air flow measurements comparing the HF impeller to the Rikon, and the numbers were significantly increased with the Rikon device. They also discussed that they had to take the unit open and may have had to sand the shaft down slightly to fit the impeller properly (not positive about that). They did mention is was a tight fit. They also mentioned that the run up took a little more time, and it definitely took awhile for it to come to a stop. I'm not much of a mechanic, and I'm very satisfied with the HF performance as is.
The more I do, the less I accomplish.