Personally I see no benefit for model airplane engines. The only things I have seen that cryogenics actually improved is the tone of certain musical instruments (this is highly subjective of course) and the hardness of ferrous parts like crankshafts and brake discs. In auto racing it has been proven that PROPER cryogenic treatment of brake discs will make them last longer. This is very important in 24 hour endurance races. Also, properly treated crankshafts will have bearing journals that resist wear better. As far as actually strengthening or tempering metal the jury is out on that as opinions vary from person to person. The only hard facts that I have read in aviation and machining industry trade magazines point to what "ilikeplanes" said - it completes the martensitic transformation of ferrous metals. Meaning it completes the heat treating process which simply hardens the metal. Might be good for the crankshafts in bushing engines. Who knows?