So i received my 2 RGB boards last week, one for my 5200 and one for my 800XL, I was initially going to use the db9 output, but the cables I purchased from Retro Access ended up being for a different revision and did not work and since both of my systems were already modded for svideo, I decided to cut the db9 connector and use an 8 pin minidin instead so I would not have to make any additional holes in either case. I'll rewire the cables down the road sometime.
In my 5200 I just stacked two pin headers in the GTIA slot to clear the chip in front of it, in my 800XL I unsoldered the GTIA pin header and used the one that was provided plus I unsoldered the chip in front of it and soldered it directly to the board without its header for clearance.
Have them hooked up to my OSSC, both work great really happy