Have you ever looked at the collector bikes on the C14? Give them a look. Then, go look at the size of collectors on just about any 1000cc bike- you'll notice that stock C14 collector pipes are tiny. Pipe size should go up with engine size and the collector pipes on the c14 are similar to the size on my old GS550. Now, I am sure many are happy with their slip on can. It looks pretty and the sound might impress some. But as for performance, get real! The stock collector pipes are restrictive and will limit power. I see no reason to pay money to buy muffler slop on bling when the collector pipes are smaller than on my GSX 1100F!
+1 on the "get real!" part.
When I decided to get a slip on (I got a Remus Hexacone, BTW) I knew perfectly what I was getting: fantastic sound (to my taste), beautiful looks (to my taste), 10 pounds lighter than the stock, and... ahem... 0.3 HP (yeap, that's less than half HP) gain, according to Remus webpage, which is supposed to be manufacturer-optimistic. I get more performance gain from wearing thinner socks.
Still, I'll do it again.