First off, welcome to Subsim.
Are you aware that there are 2 main.cfg files? The first is in the main SH3 program folder, the second is in the respective career folders in My Documents. If your career is already established this is where you will need to make the change.
Lastly, you might want to make it a little easier on yourself and use SH3 Commander which has many other tweaks in addition to time compression changes.