Slow down retronerd! Let's take this one step at a time. That way we can make sure everything is understood and works properly...
Calamity has given you some good info. Spend a few moments to digest that before jumping ahead.
Here are the basic steps for arcade perfection using GroovyMAME:
1. Uninstall Soft-15kHz (you won't need this anymore).
2. Install the custom ATI driver (you've done this already).
3. Edit your vmaker .ini file to point to your MAME .exe file. Set it to generate an MAME XML the first time you run. Leave the monitor type as "CUSTOM".
4. Run vmaker and then restart your computer.
5. Launch Arcade_OSD. You should now see a HUGE list of video modes. Far more than with Soft-15kHz.
6. Place GroovyMAME.exe (or similarly named) in your MAME folder. Backup your old mame.ini and then generate a new one with GroovyMAME.
7. Launch GroovyMAME and choose a game to play...
If all the steps were followed correctly, you'll now be playing your favourite (15k) game at native resolution AND refresh rate on your Sony SCART television. No need to make a custom resolution for Mortal Kombat, Street Fighter or Pulirula... GroovyMAME and vmaker do all the the work for you!

As Calamity said, if things aren't perfect already, you can work at discovering the upper and lower ranges of your monitor and create custom "monitor_spec" definitions to help with sizing and centring. If you re-read this thread right from the start you'll pick up most of the steps involved.
In summary, GroovyMAME, vmaker and Arcade_OSD are a suite of tools that work together. You don't need Soft-15kHz and should run the GroovyMAME build instead of regular MAME if you want CRT nirvana.