no decent results on games that don't already support progressive output from within the game. better picture this way, better upscaling depending on which converter you use. more options for screens to use, you could go rgb to hdmi potentially.
more random 15khz rgb to hdmi google hits? narrowing the focus now just a little bit
http://m.ebay.com/itm/15Khz-RGB-RGsB-RGBS-to-VGA-Scan-Line-Doubler-Converter-/310413209763
GBS8220
this converter keeps popping up, but now youve doubled the size of your portable haha...
I am not good for much but googling, but I picked that converter because it can specifically convert 15 hz interlaced to progressive. Flipping through a couple pages and through a few online stores didn't yield any type of converter that wasn't happy with a 60 hz signal.
question about interlaced video, could someone buy or build your 15 khz board and then buy or build an interlaced-to-progressive adaptor to get better video signal for a regular ps2?
a converter like this maybe?
http://www.ambery.com/rgbcgatovgac.html