...How many do you have now? Can't the new fans use the old connectors? Bottom line - you can't get a quart out of a pint pot. If you've noticed the fans slowing down, there may be one of two reasons:
1. The PSU cannot supply enough power for them all to run at top speed.
2. The fans DO NOT need to always run at top speed. They are being regulated by software and the system is cool enough.