Yes, small writes are an issue. Basically SSDs have to write whole erase blocks, not their parts. Erase block has usually 2MB. Sometimes 8 and I even heard a suggestion that one drive might have 16.
(I don't remember how many drives do you have, for this post I assume 9 in RAID 5).
When you have 64k stripe and a 0.5 MB file - it gets stripped into all drives. And you write total of 18 MB with performance of a single drive. You get performance increase when file size exceeds 2 MB - all drives write just one block, instead of one drive writing 2.If I get it correctly (which is not that sure), write performance should be about the same all the way up to stripe size=erase block size. Actually should get better slightly because controller has simpler job. Life expectancy would be the best in this case too.
You'll loose read performance though.