In all testing and observations, the volume capacity of SSD drives does not effect the performance. The Garbage Collection efficiency however does. that is where you see SSD drives lose their performance over time from writing to the SSD constantly. If this is a cache drive, it is resolved by running a clean all command on the drive on a regular basis. However if this is the OS drive then it becomes more complicated. The firmware has to reflect the efficiency to keep the SSD drive close to maximum original transfer rate capability. However SSD drives will always be faster than mechanical drives because of their latency. SSD drives measure latency in microseconds. mechanical drive latency is in milliseconds.
My son used his hard-earned money to buy me a 120 GB IBM 520 series SSD as a present. He surprised me completely! It's now the OS drive for my Windows 8 installation, and I plan on using that drive until it drops. I need to fill it up to about 80% capacity for my critical apps and Windows, and I may approach 90% with some "nice to have" tools. I won't worry now about doing that. I'm impressed with the drive so far, and I've told my son that more than once!
I have two Crucial M4s in a RAID 1 that must be around 90% filled or better. They work just good as new to my subjective eyes.