I only know what I read.  Most guys that have used them seem to think the front tower brace has little effect and is mostly for looks.  If you really want to stiffen the front get the x-brace that installs under the car.  In our DIY Library sticky there is an install for the x-brace.  Some guys have hood clearance issues on the strut braces also.  BMP Design makes some nice braces.  Now, on the rear most of the guys seem to think that they help, but you do have to cut and make allowances for the trunk liner.

I may have forgot the E46 part.  But I would think there is option for M3, it might retrofit.

I checked the DIY Library and there is a link in there to the E46 DIY Library.  As you thought I didn't find a reference for a x-brace for the E46.  There was a article about installing an OE strut brace on a E46 M3.  I didn't read it all, so I'm not sure how well it will work, if at all, for a "regular" E46.