I'm not sure the success rate, but I do know several people with the conversion and they are all happy with it. The Bekker 2.6 conversion is good because it takes the best of the 2.3 engine and combines it with the 2.5 engine. There kit is better than a 2.5-16 alone. If you think the 2.3-16 has no low end power, drive a 2.5-16. Bekker's conversion helps the 16V have more power through the entire rpm range, although peak power is still at very high rpm's. The Bekker kit can give you around 240hp in a street legal version. If you go with a non-street legal version you could get some more power. As for the engine potential, I would say around 350hp from the engine is a much as you could do (no turbo or nitrous). This is inline with the 190 DTM car, and what AMG is going to do to my 16V (when the stock engine tires).
MBCA, PCA, BMWCCA & SCCA* member
86 190 2.3-16
83 944 ITS Racer