Does the SSD actually use this unformatted space? I know that SSDs from the factory have a lower available space than their nominal space, to account for the eventual dimishing quanitty of functional floating gate transistors (ie 240GB is made available in a 256GB drive), and that OSes usually recommend keeping about 10% of the formatted drive free (for paging iirc). Didn't know that about unformatted space.
All clean pages (from overprovisioning, unformatted space, and trimmed space) should be treated the same by SSDs. In particular, Intel documents that leaving unformatted space will increase performance on their SSDs.
I may be wrong, but I'm fairly certain that on SSDs the wear leveling is done over the entire flash capacity. the details of where data is stored are an abstraction layer on top of that unlike on a spinning metal hard disk where specific regions of the drive are allocated to particular partitions
Wear leveling is done in the layer that translates between LBAs and actual physical chip numbers and addresses. Drive partitions are well above all of that in the abstraction stack, and partition tables are totally beyond the scope of a drive's concern (even for spinning rust drives). These days it is even common for a drive to have two different partition tables in two different formats with different partitions listed. Bootloaders and operating systems are the only things that should care about partitions.
This is also true for hard drives. Fragmentation and filesystem layout make performance of a 95% full partition on an HDD much slower than a 50% full partition.
Defragmentation and cleanup on traditional hard drives take longer when they're full for different reasons including the increased amount of data being managed and in many cases empty space being further away from the data being moved which will slow down moving of data to clear and consolidate free space. On a solid state drive the bigger issue is that there is a minimum size block of data that can be written in one operation. Having empty space available means that the drive can write data into that empty space without having to do garbage collection first.
Perhaps 8.1 and Windows 10 are better about what they save, but going with anything less than 128 for Windows + apps + data seems risky to me.