I really don't think it is your carburetor adjustments. If you can at least get it running and not stalling, you should see much performance than that. At the richest setting on my MT2 that I can have and it will run (though barely), it will still hit around 20 MPH.
The slipper clutch IS in the manual. The instructions to build it (or, take it apart if you follow the instructions backwards) are on page 15 and the adjustment section is at the bottom of page 11. Check both of those and see what your slipper looks like. I have mine set at "rather snug", as in somewhat tight, and it seems to work pretty well. You'll just have to experiment with it.
As for your noise, there is one part that will really increase the noise, and the performance at the same time! It isn't a hard install, and if you decide to get it, I'll walk you through installing it. Here's a link: [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXSP47&P=RF]click here[/link]. You may also need [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXDDU1&P=RF]this[/link] and I think [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LX2476&P=7]this[/link] will work to hold the pipe on. I don't know what kind of screw head it has (Philips head, flat head, allen screw), so make sure you have an assortment of all of these before you try that. Here's a picture of how I have mine set up, I have a screw through the chassis, a bolt that doesn't fit the screw (too large) under to boost the height of the hanger wire, and then the hanger wire is set on top of that, and then a nut that does fit the screw is securely tightened on top of that. I also had to saw off part of the hanger wire using a Dremel tool (if you don't have one of these yet, I strongly recommend getting one; they are VERY handy) and bend it using some needle-nose pliars and a vice. It wasn't hard at all to do, just make sure you don't cut it too short. But I saw a great performance increase and it was relatively inexpensive, and also a lot louder too. It is definitely louder than a weed-eater now. Much louder!
If you need any more help, just ask! I'll be happy to as will everyone else on this forum.