The site uses shares submitted, which isn't really a reflection of your "hashrate" reported by your miner - it is what your miner calls work units. On my own Gridseeds, I've noticed that if I use higher difficulty I see a big variance in my shares submitted per 10 minutes. I use 128 for my 6MH and 10MH rigs of Gridseeds. For less than 16 of the 5-chip Gridseeds, I would use 64 for your difficulty.
Thanks for this info. I think it might be a good idea to update the difficulty section of the help page on the website, as of right now it suggests significantly higher levels of difficulty than what you gave here.