Sure, I do it, so can you. The optimization happens in the mexopts.bat file. If you have R11/R12, issue the command prefdir(1), which will reveal its location. For R13, I hack on the mexopts file located at c:\matlab6p5\bin\win32\mexopts\df66opts.bat. If you do this, be sure to save a backup of the original! And if you use R13, you will need to run mex -setup after each change.