Sure you can uninstall SP1. Unfortunately, I can't guarantee that your game will work with the mods once that is done.
The Quadro line really isn't really for gaming, as you know. I would suggest a high-end GeForce 8, 9, or 200-series or high-end Radeon card as a replacement. But if you need the power of the Quadro and want to do gaming, I think you can have both a Quadro and a Geforce card in your system on separate displays. And no, I would not recommend SLIing a Quadro with a GeForce. The drivers for the 2 would cause you endless problems (considering you would need 2 drivers to run the cards). I won't recommend the Quadro for playing anything but Shockwave/Flash games or Minesweeper (and the like).
For Nvidia cards, you need a driver that supports both cards to work properly. Quadro drivers don't work for GeForce cards and vice versa.