Playing media over an network? Probably 1GB Ethernet? That is about 125Mb/s max. The 8TB (Seagate Archive I assume) reads like crazy until the last, say, 2TB. IMHO< read striping is only sensible if you have heavy (read) I/O on the machine and streaming a video ain't that.
Also, I'd keep balancing as default. It will first fill up the 8TB HDD (most space free) and the other copies will be dividided over the 2 2TB HDDs (it needs to go there due to duplication and will select HDD with most space free).
I think, once you get to 4TB of data, DP will not allow you to write data to duplicated folders, sort of Disk Full, and at least report errors in duplication status if you force more data on the 8TB HDD. Christopher will tell.