I have changed my mind about the size of LV's and decided to go with 8GB instead of 4GB so I don't have to guess about having enough space in /*/tmp to compile a new kernel. I'll edit this post to add my results when I get done. Hopefully that will be within the next 3 hours.
Well, I have run basic tests with defrag and zlib compression and changed RAID back and forth from 0,1, and 10, added large amounts of data, deleted some of it, did it all again, made a few snap shops, deleted them, and so on.
I can't find any obvious problems with the usage or functionality of kernel 4.9 on a btrfs filesystem created with newer kernel. I know that data point doesn't make a statistically useful sample, but it does put my mind at ease.