I saw the thread and wanted to chime in and give my opinion.
Firstly, I did my best to listen to as many soundbytes on youtube of the different slip-ons. (I'm not willing to spend enough to get a full exhaust). The Cobra slip-on is probably the most common. Personally, I think all are of comparable quality, but just have different characteristics.
I ended up getting the Rush Slip-on for my 950T. I ordered mine with a 2" baffle. Rush (and I would imagine there are several brands with interchangeable baffles) lets you choose a baffle size to customize your sound by the sound of the baffle size. I think the sizes available for Rush baffles range from 1.5", 1.75", 2", and 2.5 I think. The 2" size I have is pretty loud. I really like the sound, but it is a good bit louder than stock. Right now I'm looking for some "baffle wrapping material"--google it... to tone down the sound just a little bit. I read the difference between 1.75" and 2" baffle sound-wise was neglible, so I went for 2".
As for performance, I told myself not to really expect anything, but it seems like the bike does breathe better and has a little more low-end grunt. I have a K & N air filter on order, so the bike should breathe even better soon. I do have some popping on deceleration, but to me it just adds to the character of the bike. With a slip-on (and I'm sure even more with a complete exhaust upgrade) you can actually hear the bike and it's really cool. IMHO. I did not want to "drill the exhaust" though I've read many posts about how happy people are with that MOD.
Some of the top brands I found:
Cobra (most popular)
Roadhouse (seems to be the loudest)
Rush (Rush seems to be somewhat popular with HD guys)