[JiveTurkey]

I haven't looked at that particular wiring but I would think the amperage has to suffer if you are adding things that weren't designed to be there. (Voltage isn't the only measurement to be concerned about.)

The difference might be small enough where it doesn't matter but common sense is that you can't repeatedly split a wire and keep the same amount of electricity through it for everything connected. The fan speed could be showing an increase or decrease because of this.

I don't really see a need for more fans but if it's already installed and working I'd just go with it.