Correct. Here is basic rule that all emissions has to be same and power can not be raised more than 20%, and if you raise power that 20% you have to have all brakes, suspension etc. similar than model series most powerful version. In z4 case 3.0si.
It is so sad thing because i have 2 M54 based cars and there are world full of aftermarket supercharger kits just basically bolt on, but is illegal to install.
Some people bolt those kits and just drive, then take all supercharger related parts off for mot check. So much work and hassle. And if you drive accident and have illegal supercharger insurance company dont cover anything. It can be really expensive for you. You in worst case scenario pay it for you rest of your life.