I switched to 255's on all four corners for purposes of auto-crossing. You can check with the people at the auto-cross events and see their tire widths generally are on the wider side, not the opposite.
I believe comparing an e9x to a miata for purposes of tire analysis is a stretch. I would bet a miata, with its reduced weight, torque, wishbone suspension and lower stance, in contrast to a 335i, probably does not need as much tire width up front. It is probably the case that increasing the width did not add any additional benefit and reducing the width to its minimum necessary to provide for precision turning was sufficient. I wonder if there are any written reports out there analyzing tire width factors? Would be interesting to read more about.
Here is an example of a racing outfit that found larger width of tires in the front reduced understeer and improved track performance in a 135.
http://www.evolutionracewerks.com/?q=node/55