Especially on soft body tissue there isn't enough diameter, expansion or mass to make any difference between a hit from a 9mm or a 40cal. Meaning you will not drop a target any faster with a 9mm or a 40cal. Even using the theory that bigger holes are better, the 9mm and 40cal are so close that it also will make no difference to your target.
The 10mm was suppose to replace the 45cal & 38spl for government agents and you are correct about the troubles they had with the 10mm's power being to much for most of the shooters to compensate for. But the agency's went back to the 45cal, not the 40cal. The 40cal was basically developed as a wildcat cartridge 10mm kurtz and was pushed into the mainstream buy the ammunition industry and several firearms makers that had firearms in the works aimed towards the big money, which is large law enforcement contracts.
There is nothing wrong with a 40cal, it just isn't better or worse the a 9mm. Just a different answer to a problem that isn't really there. I suppose if I was new to shooting or didn't already own so many 9mm firearms, I may have jumped on the 40cal bandwagon too.
I don't mean to sound critical of anything, just being informative
