I've added a relative speed attribute to CCParticleSystem. I'm still using v1.1 in my 'should have been finished long time ago' project.
I just scaled dt with my relativeSpeed attribute in the update method. My game has slow/fast motion replays, and in those replays I want the particle systems to slow down or speed up. Maybe there's another way of doing this, I don't know, but if you like you can add this feature to future versions of particle systems. It's royalty free!
Does anyone see any risk with this modification in CCParticleSystem?
-(void) update: (ccTime) dt
dt *= _relativeSpeed;