Here is what an engineering friend said about the QuadroPlex;

Given the hardware architecture, you pretty much get what you would expect.
Operations that require downloads from host memory to the QuaroPlex are
bottlenecked by the single PCI-16 connection, but peer-to-peer operations
(board to board on the QUadroPlex are very, very fast. So, If you custy is
running a flight simulator, they are better off with the SLI, if they are
doing screenspace composited volume rendering, or anything where multiple
cards composite to one display, then the QuadroPlex is the ticket.

He IS running flight simulators, so he feels it was a waste of his money. However, it is useful for development OF that simulator. Here is his site so you have an example of what he is doing;
www.andesengineering.com (although it appears his site is down at the moment. I left him a message letting him know)