Well, then... The only advantages aside from the processor would be a standard Airport card (if you don't already have one), a faster GPU and a slightly larger standard hard drive. You also get faster memory (1333MHz DDR3 versus 1066MHz DDR3) on the 6/12 core models.
But unless I'm missing something, I think that's it.
Firmware. And it's very likely it's the daughterboard that you'll need, not the backplane board (one that has the PCIe slots on it). Please keep in mind, this would only work if none of the hardware's actually changed (no part substitutions anywhere that would affect the ROM). This isn't likely, but without confirmation, the mention of this condition is necessary IMO.
SP daughterboards can be had for $350, and the DP versions $400 as I understand it. The backplane board is $300. This would work if no means of flashing the ROM becomes available, and Apple doesn't release it to 2009 system owners voluntarily.