Using a defouler on e90
 

Will a defouler work on the e90 when using catless down pipes vs using DP-Fix's

Does it deepen the sound

Defoulers will not work, the DME is too sensitive. Besides that, there's not enough room to actually install them anyway.
Keep in mind that the dp-fix is a fiddly thing and BMW uses all 4 O2 sensors for AFR calculations

I agree there is barely enough room for the secondary O2 sensors as is.

Historically they do not work on this platform.

They do have a 90 degree defouler and I hear they do fit whether or not they work is another story.

Even if they do work I dont think it would be recommended if you keep them in for any length of time. If the DME is using values from pre / post cat O2 sensors to make calculations on AFR then using the defouler for an extended period of time may cause other issues.

You can simply code out the cat codes if you have an MHD license to read and flash .bin files.