Be prepared to do a lot of testing. I spent a great deal of time trying to understand the relationship between number of writes and write performance of the generally excellent FusionIO devices. The vendor wasn't too helpful in understanding the situation at first.
Remember that the definition of a "free" block can vary. For example, if you wrote to it ever, the storage can't consider it free. Perhaps if you later write to it with all 0's or all 1's, it can flag it as a free block although I have not seen this in practice. I found shocking performance artifacts by creating a logical volume that was only a small portion of the overall storage. Then, creating a logical volume that contained the rest of the storage, writing to it heavily for a half-hour, and deleting the logical volume. To the SSD, these blocks are of course not free. But I had forever destroyed the write performance by doing this test.
By formatting the device with extra "reserved" blocks, the reality of the performance impact was made irrelevant. The FusionIO devices support fast writes because they erase the reserved blocks in the background, and use them for new writes. (The block you overwrote is flagged as free, and later gets erased).