Is the Euro 2.3-16V having the same cams as the 2.5-16's.
from what i see the improvement that was done on the 16'v developemnt from 2.3l to 2.5l was mercedes improving the vavle train system for higher PRM limits. Minimize valve float, incresing valve spring rates, lighten valves (not sure) etc. As the engine breathes easier on the rpm's more power can be produced. Check the racing versions 10,000+ rpm.
When I was younger (10 years ago) I've seen evo's racing with e30 m3's in the south east asian supercar racing. wasn't really into cars then, but i still remember the black 190 evo's (ex-dtm works machines) from thailand almost always kicking m3's butt.
I do think the evo's are higher cost to maintain in race form further more the volume of cars produced were low. There are still a few e-30 M3's running in the supercar category in my region.