I was unsuccessful in changing the projectile speed of syringes using tf2attributes. I wrote a plugin that applied the 'Projectile speed increased' and 'Projectile speed decreased' to syringes and it didn't work. So, while those attributes seem to work well with rockets and flares it does not work on syringes.
I'm only posting this here to let people know what I tried that didn't work in case somebody had a similar idea.