The system partition is 3.2 GB. It isn't like you can go from 24 to 31 GB free. If your that desperate over an extra gig, you might need to reevaluate what you have on your phone, or invest in a microsd usb card reader.
Also, 200 MB is most likely the size of the zip file, you are using to install the rom. That is compressed size. Mahdi rom (I have on my pc at moment so could check), is 208 MB compressed.. The actual install size is 351 MB. That doesn't include gapps, which also installs to the system partion. The core gapps package is 18MB but installs to be 30 MB. And that is assuming you are using core, the full packages are MUCH larger.
So theoretically, you could free up maybe 2.7GB. Is it really worth a good chance at bricking your phone for that?