What a coincidence. I just finished dealing with my exhaust-leaking HMF slip-on. After doing a little research online it looks like every HMF slip-on for any kind of bike leaks at the header pipe connection. You may want to double check to make sure the leak doesn't come back. I tighted mine up as much as i could and it didn't help any. Here is what I did to fix my leak.
I got some Permatex Ultra Copper silicon from Auto-Zone (rated at 750F)
I also picked up a strip of VersaChem exhaust pipe adhesive wrap. It is about 2" wide and is made of very flexible aluminum. Has a paper on one side that you peel off to access the sticky side.
I made a squigly bead with the copper silicon around the perimeter of the end of the header pipe. Then I slid the HMF slip on over the header pipe end (don't forget to slide your t-clamp onto the muffler pipe before you do this). Then I ran another bead of silicon where the end of the HMF pipe overlapped the header pipe. I took a piece of cardboard and smeared a thin layer of silicon around the joint. Let it dry for 24 hours, then I came back with VersaChem aluminum wrap and wrapped it around the joint and over the rtv silicon. The exhaust wrap is really flexible and allows you to mold it to the contour of the pipe. Then just come back and tighten the clamp over the exhaust wrap towards the end of the slip-on.
I wanted to make damn sure it didn't leak because I read several posts online where people used RTV and silicon, but it either didn't fix the leak completely or the leak came back within a couple of months.
I don't remember a time when any of my quad's pipe connections didn't leak a bit - except for a few months after I used copper high temp silicone on the Rappy's... that lasted about 3 months, then leaking again.
I could feel my leak, and barely hear it only when I got close to the header/slip-on joint. I would assume that an inconsistent leak (getting worse over time) may affect the proper air/fuel mixture. I wanted to make sure mine was not leaking before I re-jetted.