For the Apple SSD drives, will they run slower with less free space available? From what I understand, most SSDs will run slower once there is 20% or less free space available, due to TRIM/garbage collection. However, are Apple SSDs the same way?

The reason I ask is because I've seen some drives that are advertised as 256 GB but actually are 280 GB or so in size, so even if they are "full" they can still run at peak performance.

So, I'm thinking of getting a Mac Mini from Apple with an SSD preinstalled in it. Would I have to leave 20% free space at least for peak performance?
The drives are provisioned with space set aside to handle garbage collection and bad block replacement. I'm sure it's possible to drive the create benchmarks where a measurable drop in performance is shown under certain conditions but I've used six different SSD's in three different MB/MBP and have never noticed a drop in performance even when the drives were almost full.