I appologize for my delayed response, been busy with work and stuff.
It IS actually a power issue, but no directly realted to the power supply but more to the BIOS itself. Basically on the 9300 the BIOS simply limits the amount of power it will send to the video card based on the card type ( x300, 6800, etc ).
Well as it happens the bios doesnt seem to recognize the 6800 Ultra, it sees it as "Unknown video adapter". When I run 3DMark05 on it and query the system details, 3DMark reports it as a "Generic VGA chipset", even though the drivers recognize it properly as a 6800 Ultra, and it reports the gpu/memory frequencies at 250/680. These freqs are about 30% than the standard 6800 freqs, which makes the 6800 Ultra on 9300 perform less than a standard 6800.
So why the 250/680 frequencies, where did these values come from? Well these are the video BIOS built in defaults for "low power consumption". The card is runing on default low power, because the 9300 BIOS doesnt know how much power it needs to run at full speed.
A 130W power supply would still not do it as the BIOS controls power distribution, besides both the XPS and 9300 use the same 9cell batteries, and Im sure the XPS can run the 6800 Ultra at top speed even while on battery.
On the north bridge heatsink, for testing purposes I left it out. However had the video card mod been successfull, I would have hacked off the heat pipe, the XPS2 has the same heatsink minus the heat pipe extension.
On a final note the 9300 BIOS EPROM is soldered on to the motherboard, so it cant be swapped. So the only other option was to get an XPS2 mb and matching power supply, the end result an XPS2 in the body of a 9300.
Well its been a fun experiment, and sadly it seems like DELL really wants you to spend the extra $1000 to get top of the line gear. Cant blame them they got a business to run, and were trying to hack around their sometimes stupid business decisions.
As for the EEPROM being soldered...it's as expected...that socket costs money you know 
