The PM9 is already quite snappy by my standards. I can only imagine the PM40 being a nightmare.
A few months ago i fired a 9mm and .40 shield together at the range. Both were using standard weight FMJ not +P. I could double tap the 9mm onto my target no problem. Switching to the .40 made followup shots damn hard for me. Im no shooting ace but I have to say the reason I dont go for a small .40 is because I think the training curve would be intense. Getting accurate multi-hits w a PM40 or shield .40 is easier said than done.