I have just finished building a 30 watt laser and it cuts denim pretty sweetly but I would try to go 60 watt or more if you want speed and clean cutting, but only if you can afford it. Air assist is essential for blowing out the flame from cutting and also for keeping the lens clean. Don't get a laser without it. Also fume extraction would be a good thing too.
The more power you have the more layers of fabric you could cut but not if it's rayon or nylon as I think it would fuse together after cutting.
Hope I am of some help
Rich.
I am not completely useless.......I can always serve as a BAD example.