Drives that are used should be TRIM-erased before trying to overprovision them. If you overprovision a 'dirty' SSD which has been written to, then you will set aside space for nothing. The SSD doesn't know you don't use that data anymore, so it is forced to maintain that data.
Step 1: clean your SSDs; perform a TRIM erase on them by running quick format in Windows 7 or later (cannot be the system disk). Alternatively, you can use different tools like under Linux or even BSD since Windows offers limited TRIM support. But a quick format should do the trick. The only condition is that you use TRIM-capable drivers, like when using your chipset SATA ports.
Step 2: partition the SSD so that a portion of space is unused. On a 128GB SSD, you can create two partitions: one 50GB partition for SRT caching, and another 50GB for normal data storage (games, whatever). The 28GB that you leave unused will be utilised as spare space by the SSD, because it knows you/the host is not using that space, thus the SSD can use it for redirected writes.
Step 3: add the partition as SRT cache to your existing HDD with Windows installation. The maximum amount supported by SRT is 64GB but you can go much lower than this. Even 20 gigabytes of cache can have a very large impact on a multi-terabyte volume.