The QuadDoublers and Quadra Overdrives double-clock the m68040, so a 25 MHz machine runs the motherboard at 25 MHz all the time and the CPU simply runs at 50 MHz only when the CPU releases the bus. This makes it so it doesn't really run at the full 50 MHz all the time, but enough time considering how much stuff can be run in the two 4K caches.
I haven't figured out a way to close the case when adding any sort of QuadDoubler to a Quadra 605, but one will fit with a good copper heat sink in a 1U case. The same holds true for adding 256 megs to a Quadra 605.
They're flexible little machines, for sure.