I have a USP Tactical as well, and was intending to get a can for it. However, mutliple RKI's have stated that it is one of the loudest platforms when suppressed - it apparently doesn't stay locked as long as other pistols, and you get more noise out of the ejection port. G21 is supposedly the quietest.
And even though standard .45 is subsonic, the larger the caliber, the more difficult to suppress. More gas is generated by .45 than, say, 9mm, the exit aperature obviously needs to be larger, and the larger diameter bullet generates more flight noise. Just about all .45 cans are required to be used "wet" to get the advertised level of suppression, and get noticably louder once whatever medium you use is gone. Most modern 9mm cans, otoh, do a very good job dry. Bottom line, subsonic 9mm will just about always be quieter than .45 when shot through cans of comparible quality.