Dreadnaught Industries does conversions, too. All the guns shown above (with the exception of the Tromix S17 SBS) are DI guns; they also make a neat mid-barrel shotgun compensator and a great AR15 comp that works very well (although, of course, I am biased). Their turnaround is usually under a month, match schedule permitting...darn competition shooters

The Polychokes work pretty well...they are not as consistent as regular screw-in chokes, but they are a lot easier to change on the fly. My wife, the multiple-year Texas State Lady's 3-Gun Champion, uses a Poly on her S20. I prefer regular (internal) chokes, although all I ever seem to use is LM (the same as on my Limited guns. If you don't want to spend the $120-150 getting tubes installed, the $89 Poly is a very cost effective way to go. Competing in any real matches with no choke can be an exercise in futility.
Alex