Why flash to X800?! FireGL is a much more expensive card. Why do you want to downgrade it?
Currently you have 12 pipes. To get 16 you need either: a laser-cut repair (see X800 instructions, it is the same technique). Alternatively you need to change the BIOS, and put a V7100 onto it. The V7100 has got 16 pipes open. Check the core ID of the V7100 first. Needs to be R423 also for the BIOS to work on your V5100.
If you do the flash, your cooler may not be good enough. Get an upgrade before playing flash-man.
Also post your fuses data. A "wise guy" might be able to add further comment (e.g. on-die pipe lock).