When I bought mine, the guys at the shop said the PM40's had problems, and pushed me to the PM9.
I'm intrigued by the PM45, and wish it was legal in Kali.
I'd have a look at Kahr's website, and their links to gun reviews that will give you how the ammunition works out of each model.
It's more about powder speed, and bullet weight then caliber in what looses velocity, and what doesn't.
If the ammomaker tailors the round to short barrels, it may maintain more speed then
other rounds. Be aware that some companies market "short barreled" loads, that are actually matched to longer barrels, using a slower burning powder. Doesn't stop them from misleading, and mislabeling the ammunition to sell it to you. Buyer beware.
Look at chronograph results, then make your choice.