I'm using a Bar-Sto conversion barrel (40 S&W to 9mm) should I also change the recoil spring?
Keep in mind that the slide on the 40 is slightly thicker and heavier.

buckshotshorty, You probably would not have to change out the recoil spring. I converted one of my 40S&W 229s to 9mm with a Barsto conversion barrel and it worked just fine with the 40S&W recoil spring.

It would not be a bad idea to get a spare guide rod and 9mm recoil spring. I have noticed better ejection, higher and further away after picking up a 9mm recoil spring.

I would occasionally get a few cases in the face when using the heavier 40 recoil spring....after going to the 9mm spring the issue seems to have gone away so far.