Keep the mtech setup!
By far the best overall for both handling and comfort. I lowered my 1991 convert with 17" Alpina replicas 40mm and couldn't wait to get back to the original mtechs. Unless you are prepared to fit really heavy duty inserts to stop any body movement your tyres will clip the arches, as mine did, if you only replace the springs.
I now have standard 325i inserts, which by the way are the same part number as the mtech setup, and mtech springs again. The mtech setup is all in the springs, therefore once you change the springs for a lowered variant you will find greater front end movement (almost like a bouncing feel) as you lose the ride height and stiffness also.
I also have a 325i sport standard and the difference was really noticeable when i had the 2 variants.