I have the kevlar ones and they were an improvement over stock. Other people I've talked to said that they didn't notice a difference between kevlar and steel braided. This was all on honda RC51's, so I don't know how much good that does you.
yeah, kevlar and UV rays don't work well together. kevlar may be touted as being stronger than steel, but it just doesn't hold up well. Unless you're racing your bike and plan on replacing them every few years, I'd say stick with braided stainless.
I use Kevlar brake lines on my Aprilia, I didn't noticed a difference from stock, but don't notice much of a difference with my buddie's RS250 with SS brake lines. Oh, and I use Fren Tubo brake lines, none of you guys have heard of them though I'm guessing (I hadn't heard of them either before I got to Italy)