I don't see why you can't use the same rotors you have (unless they are bent or contaminated)? You can also most likely use the existing adapters to keep a 185mm up front. When you take off the old Avid Juicy 3 caliper it should be attached to the 185mm adapter. Simply leave the adapter in place, and screw on the SLX caliper. This will only work if both Avid/SLX calipers are the same design or if the adapter is multi-purpose (post mount or IS). Basically the screw holes should line up logically..if they don't then you need a new adapter probably.

Moreover, the front brake is capable of more work than the rear it just takes a little more skill to properly use. This is why you had a 185mm up front and a 160mm on back.

Performance difference will most likely mainly be felt at your brake lever. Almost any decent disc brake is capable of completely locking up the wheel when set up and maintained properly. What you will notice is that you will have more control over when or IF you lock the wheel locks up, and you will have to most likely exert less force on your lever for the same performance. It should feel a lot better.