"Any other not-kit items you bought and used/tried for this hell?"
Sorry, I realized I forgot this question too.
I didn't modify this kit other than a carbon canopy and a carbon tail boom.
Thinking about getting one or both Mikado 90 degree antenna brackets to experiment with. Also, not sure if my kit came with the new flex skids but maybe those too. My radio had a faulty potentiometer, glitched and resulted in a hard-ish landing but no damage so I'm probably okay without.
With that said, I pretty much modded every other kit in the past, some even before the first flight. I've made general reinforcements, upgraded hardware, removed things for weight, moved things for weight, etc., etc..
I just haven't found anything that needs much extra attention on the 516. I really like the tail boom doesn't require struts and the battery tray is pretty easy to use with a little practice. (I bought an extra one) and everything is pretty much mounted as high as it can go.
Random notes: I do find the servo arrangement to be odd but it works. The belt tensioner is also odd but it also works. I think it works because it's dynamic and ever changing so don't be surprised to see it move. I think a better, less exposed spring tensioner is probably best for the future... like a serpentine belt style tensioner.
Hope this helps! Let us know when you make your purchase.