I love my Evo-40/G35 combo. I was at the 2007 AAC Silencer Shoot a couple weeks ago and was shooting it next to a guy with an Evo-9mm and another guy with an Evo-45. We were all running our cans dry. I noticed that the 9mm was slightly quieter, but it had the same tone/type of sound. The .45 was a whole different story---the tone was different and it was way louder, almost painful.
The biggest difference between the 9 and the 40 is when the bullet hits something.
ETA: I use WWB 180 gr. It is comfortable to shoot dry without earpro. When Al Paulson did a review, he got 132 dB dry and 120 dB wet on a G22. The Evo-9 got 126 dry and 120 wet, and the Evo-45 is somewhere around 141 dB dry.
It makes a huge difference where you shoot. The first time I went out, I was shooting it in a wooded area at a pond bank 20 yds away. There was a whole lot of echo, and the bullet hitting the target was really loud. At the AAC shoot, the targets were about 100 yds away and we were shooting in an open field---the sound was completely different, almost "hollywood."
You don't need to change the factory spring btw since it's a boostered can.