All SSD's have some over-provisioning built in. Because Flash memory is produced by the GiB but sold by the GB. That difference gives some over-provisioning for the SSD controller to use. The likes of Crucial etc typically sell a 256GB SSD as a 250GB model. Using that 6GB space as a reserve and extra over-provisioning.
The question becomes is that enough? Well it depends. If you have a more expensive SSD with a DRAM cache, in pretty much all situations I've encountered there really isn't a need to partition your SSD with an unformatted space left as additional over-provisioning.
For the cheaper SSD's, particularly the ones with only two chips typically under 500GB capacity and without a DRAM cache, its not so clear.
Light use as just a store for games or whatever you probably don't need to add extra over-provisioning.
For any sort of heavy workload, or in situations were the SSD gets filled to near capacity, or for a boot drive. I would recommend adding additional over-provisioned space.
For example my old 128GB Sandisk Plus SSD was used for a Windows boot drive for ~3 years. It was often near capacity and would suffer ~2000ms response times installing updates and write speeds were down to ~60MB (slower then a HDD!). I had to copy all the data off, and run "Diskpart clean all" a couple of times to retrain the SSD controller and suddenly response time was back under 100ms and write speed was up to 300MB/sec.
Reformatting the SSD i created a partition of 110GB and left the rest as over-provisioning. Even running near capacity the SSD remains responsive and writes quickly.
There is one other case I've found were over-provisioning helps all SSD's. That is old SSD's that have less then 100% remaining life. Quite often you will suffer a system freeze on writing as your old SSD will stall because another block has failed and there are no spare blocks to replace it. This is happening on my Crucial BX100. Think its around 93% remaining life and often it would cause Windows to freeze. Over-provisioning additional space has fixed the freezing problem and I can use it normally until its next upgraded.
