Well a group of friends of mine and I were talking about different size tires on the same bike.
I had no experience in this matter myself but thought it was interesting.

Mixing tire sizes is clearly nothing new, but is it worth it?

I live in a hilly area with both cracked up and smooth roads, so i chose Ultra Gatorskin's in 700x28c for my bike to get some comfort when i was on the "*****" parts of my road to work.

Now I'm wondering, since my rear wheel is already pretty beaten up and the front looks like new, if I would feel any difference by changing my front to a slimmer more race style tire, and save the front Ultra Gator for a future rear tire.

I like to go fast, so the only thing I really want to know is if I will experience any increase in speed (and don't loose comfort by keeping a 28c rear), or if the difference between lets say a Grand Prix 4000S and my Gators is so small i wouldn't tell the difference anyway (23c GP4KS/28c Gator vs. 28c Gator/28c Gator)?

You are running 28c tires. So, maybe 70-80 psi? 23c tires are going to be 120psi or so. You bet you will feel a difference. Will you be faster? No.

the GP 4000 has less rolling resistance and is more of a race tire, so it feels nicer. If you're riding on really crummy roads then stick with wider tires.