Yep, I have. Mostly highly stressed gears and misc. motorcycle engine parts though. I've not had a case done. Not strictly relevant to motors, but we had an issue breaking transmission gears in a race car-literally went through a 2/3 gear pair every 20 hrs of run time. We decided to have them cryo'd (shop below), and went over 100 hrs without a failure. We actually switched engine types before breaking another gear.
Talk to Gregg Hekimian at Hekimian Racing Engines (Watertown, MA). He's quite the character; very smart and super nice; plus he's been doing this quite a long time. Parts generally have very fast turnaround and treatments costs are pretty good (IMHO). I believe much of his business is actually in coating for Aerospace applications and the engine stuff is something of a side/hobby business since he only does it on the weekends IIRC. My friends and I have used him a few times and been very happy every time. Once he actually sent back a check for something like $1.50 because we over payed (wrong weight by a few ounces on a big order).
http://hekimianracing.com/cryogenic-servic...nic-treatments/