Started doing some tests, I'm going to change the methodology, because in my thinking if I only over provisioned one of the SSDs the test can be limited by the parity SSD, since it won't be over provisioned, any performance improvement may not be visible, so I'm first testing in a 3 SSD array, all using full capacity, then I'll over provisioned all 3 and repeat the tests.
P.S. Now that I want them to slow down they haven't yet, I'm using 120GB 850 EVOs and I've written more than 1TB to disk1 and it's still writing at max speed 157MB/s (150MB/s is the announced max sustained writing speed for this model, also for anyone interested if I use normal read/write/modify instead of turbo write, write speed is 112MB/s due to the parity write penalty, so as expected parity penalty is much lower than with HDDs )