A 5 inch Cat performs like a 90mm refractor.
I'm not sure I'd completely agree with this - exactly, anyway. See below.
There is a review somewhere here on Cloudy Nights comparing the 102HD to a Cat.
Actually, I think you may be talking about my TV102 vs C5 comparision. The c5 and the APO were very close on most targets. The 102 had the edge on everything but resolution. In resolution (on globulars in particular) the c5 won.
If you are going to extend this to an achro, don't forget to take the deleterious effects of chromatic abberation (particularly at higher powers) into effect.