You could run the 2.5" converters, but everything in front of and behind the converter (until the pipes join) is 2.25". It would be easy enough to change everything behind the converter to 2.5", but I'm not sure that it would be worth it unless you increased the pipe diameter (headers?) in front of the converters. I think that M Wong (E38 RF) went with the 2.5" converters, but had trouble down the line. See if he has anything else to provide.
Have you considered measuring backpressure while under load to see where you are currently at?
bimmerfest 5-Series Welcome
E39 - 1998 540iA Sport (11/97 Production)
E30 - 1989 M3 (44k miles)
1979 Triumph TR7, still have my first car ;-)
E21 - 1980 320i (sold at 130k)
E23 - 1986 735i
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