well - this driver does not offer that many improvements to the kernel any longer.
Also it is not maintained for 3.18 any longer.
But on the other hand there are several improvements that possibly will go into the 4.1kernel version, which the foundation might want to back-port to 3.18 at some point.
The big question is: are you running the driver in your own code (using spidev) or do you use a kernel module?
If you use spidev, then you might want to think about using the SPI-device directly from userspace, which avoids the context-switches and interrupt latencies at the cost of CPU utilization. I think there is a library out there that can help you with that.
But I guess with 4.2 there might also be some advances included that will improve spidev - for short transfers it would even be compareable to what the user-space library does...