I'm not familiar enough with exec-stream and node-brake to understand all the avenues for data-loss.
However, I did a little experiment to see if node-brake has a back-pressure effect, which you mentioned could be a potential area for data-loss.
File is also hosted on Gist.
Results of the process are below.
I notice that there are no gaps between transformations before the brake. However, the brake disrupts transformations later down the line. Given this data, I suspect that node-brake has no back-pressure effect.
[Screenshot of console output generated by my sh2png utility]