I have the Clark Custom kit installed in my Kimber Tactical Entry II full sized 1911 (originally for the .45 acp). The Rowland makes the gun a bit heavier and the muzzle break combined with the weight seems to make the gun more stable, and thus very accurate. It's also a very impressive looking combination (to the extent looks are important). I don't have a chrono but from the loading data (my son reloads) and from the felt recoil I'd say the rounds perform as advertised. By the way, the load is a handful - much more felt recoil than either then .45 acp or 10mm.
A negative (besides recoil): Because the slide spring needs to absorb so much more energy its a much stiffer spring (24 or 26 lbs?) which makes manually cycling the slide or performing a chamber check a more challenging excersize. More importantly, simply installing the spring (originally and after cleaning) is a challenge. It took me 20 minutes the first time as I just couldn't get the darn thing compressed! It gets "easier" with practice, but never easy. I'm looking at getting a graduated spring to help with this process.
In this pix is the Kimber with Rowland kit, next to a Dan Wesson Guardian 9mm.