FSX reqiures 140GB? Woah, that's a lot of textures.
Filling up a SSD in itself does not decrease performance. It is when additional writes need to occur and there are no clean blocks left. The spare space helps to ensure the SSD can maintain enough clean blocks before encounter write performance degradation.
Games mostly read data and perform relatively few writes. Being the case, ~10GB spare space should be fine to allow the SSD to have spare space.
If you're worried, run a benchmark on the 160GB and pay attention to the 4K Random Write performance.