There seems to be misconception about what link aggregation really is. Please refer to wikipedia and read on LACP for a detailed explanation.
In short, aggregating two 1GB ports won't give you the equivalent of a single 2GB/s port, effectively making a single connection twice as fast. It rather splits the load of two (or more) separate requests to the available ports, thus making only sense if you accessing files from more than a single machine.
With linked ports on a DS you can access files from two machines with 1GB/s EACH. A single machine will still only be able to access files with 1GB/s, even if you link the ports on the machine as well.
For a single connection, 1GB/s is as fast as it gets. In my home network, I setup my 1512 with LACP enabled on a HP 1810 managed switch and can access files from a variety of machines and TVs simultaneously without being limited by a single 1GB/s connection that has to serve all clients. Instead, the DS uses both ports to serve the clients. That's what LACP is for and nothing else.