My Soft15khz experiment, advice needed

Hi all, firstly a big thank you to SailorSat for his efforts with this program. I am not a CRT owner but I decided that I would experiment with Soft15khz because I had all the spare parts lying around (compatible videocard and a hard drive to install XP to). My setup consists of an XRGB3 video processor that I use with my HDTV for upscaling SNES, Megadrive Amiga and others. I had hoped that with Soft15khz I could make the PC output at 15khz so that my XRGB3 could handle the up-scaling and add fake scanlines, since I think the XRGB3's scaling looks a lot nicer than emulators normally do.

After some experimentation I got things to work, and the initial results were very pleasing. However when things start to move, its just not very good. I don't get screen tearing after tweaking emulator settings as such but, if you watch the edges of platforms for instance, its as if they are tearing ever so slightly as you approach them, the scrolling just doesn't feel smooth in games. I noticed in MAME I had to turn on triple buffering to eliminate screen tearing, which I did not expect. I've used this setup with a lot of real 15khz machines and never experienced anything like this. Is it just a limitation of using emulators or is there anything that can be done? Is it refresh rate related perhaps? Any ideas anyone has would be welcome, thanks.