If the 2000 pad is a new (or newer) one you should notice a difference.  However, first take it down to 500 if possible, then skip to the 2000.  Merely hitting it with a 2000 would not make much difference in my experience.

Thinking of taking my RG Menace from the OOB (3000) to 2000 grit for Nationals.  Will I see a noticeable difference between those two surfaces or am I wasting my time?

It depends on how many games you have on this 3000 grit surface.

If you have 6 or more games and have not refreshed the surface yet, the ball is probably at 4500 - 5000 grit already, just from bowling. If this is true, then you will or should notice a difference immediately. If you have been keeping the surface fresh (At least every 6 games) then the situation will be as notclay describes.

Doing the 500 than 2000 will give you the teeth under neath where it will still read the Mids and still be strong that's the way I always go