It is very rare that an engine shop can produce 500 SHP out of a 450SHP engine on a consistent basis. This would yield +12% engine. In some cases, you may get an engine off a test cell that may produce that type of power on a run but the engine will not sustain that type of excess performance through the required hot section interval let alone the whole overhaul. If you know someone who can consistently build a +12% R engine that holds that power advantage all 3500 hours, have them email me, I will finance them in an overhaul shop.
The series II engine has been massaged, poked, prodded and modified to get to 450 SHP in the R version. Even with the enhancement kit, that engine has simply run into the laws of physics.
Ambient temperature is a problem with the R engines. In hot climates, the performance drops off very quickly minimizing the advantages of the R. It is not just SHP at sea level but SHP in the operators environment. Because R's are pushing the limit of its fundamental design, they have a much higher incidence of turbine rubs, coking and compressor stalls.
There are two basic reasons for the RR500.
1) The new compressor section is more efficient, less costly to maintain and will provide more power. This compressor section is similar to those used in series III (c30) and series IV (c47) engines. The hot section has higher TBO and re-designed blades and nozzles. The combination of these re-designs results in more SHP and much less fall off of SHP in hot or high temp uses.
2) Rolls Royce hates the PMA market as it costs them tons of lost revenue and profit. With the exception of turbine wheels, you can buy just about every other part for a series II model 250 either through PMA manufacturers or through sellers of serviceable used parts. Say what you want about a model 250, but there are tons of them in use all over the world, they can be worked on by thousands of mechanics, they can be overhauled at both RR authorized and non-authorized shops and the result is that they are cheaper to run by the hour horsepower for horsepower. So......re-design the engine, tell the FAA it is "all new" and presto, you get to re-monopolize the parts, component parts, component overhaul, engine overhaul and engine repair business for your authorized shops.
The purpose of the engine is to make a better mousetrap, but also to make one that is more profitable for the manufacturer.
But maybe I am a cynic and they are really trying to do it for altruistic reasons....