The hard drive inside the GoFLex is just that a plain hard drive. the enclosure is what houses the type of interface. and speed is not the issue since you can get a thunderbolt "adapter" which is waaaaaayyyyy faster than eSATA.
Any time you use an adapter to go from one type of connection to another you are losing speed, period. The speeds you are thinking of are when you use the correct connectors with the correct devices. Also, if you're going from a faster medium, which you claim, to a slower medium, which you claim, you're creating a bottleneck.
With the DirecTV DVRs you need to use an enclosure that already has eSATA. Not some rigged up concoction where adapters are used.