Maybe these options are ignored by the driver, I'd have to check more precisely. But it's still strange your situation changed after adding this config file (and yes, i agree you can't have >40Mb/s on a supposedly 10Mb/s negociation :)).
About the 150 vs. 350 figure, I don't have an explanation (maybe OSX takes advantage of new TCP features, which AmiTCP can't, just speculating here).
