The base idea to swap the current controller with a faster one is good and feasible.
But we must face with the processor bus speed, and see if this bus speed is enough for the new speed of the new controller.
The BUS is the "highway" where all the peripheral signals go and come to and from the processor. If the speed of this bus is equal to 20mb/sec (example for the 550D, whose SD controller is limited by this speed), then there is no point to improve the SD controller speed.
If then we try to overclock the bus speed, then the processor may lose some data from the bus, so the bus and processor speed are tigthly correlated. You can overclock the processor, but then it could behave like drunk and screw up all the system, on a random base, and it can overheat and burn on the long term. So, we can only see if the bus speed can let us to improve the SD controller speed.