Looks like detailed and reliable - altitude performances for RoF aircrafts. Charts and explanaition. I found it useful.
excelent link, thanks mate!!! 
supercharged engines are much more advanced in this point of view…once I was flying S.E.5 and was surprised how could Fokker D.VIIF climb up to my altitude of 10000ft and even close distance to start attack and when I droped down to deck, I left him way behind…makes sense