The number you are looking at is 40MB/s, i.e. 40MBytes/s. 100BaseT has a theoretical maximum between 10MBytes/s and 12MBytes/s, so if the FS can really get to 40MBytes/s over Gigabit (theoretical maximum between 100MBytes/s and 120MBytes/s) then you should see 4x the performance on Gigabit versus 100BaseT.
I get 30MB/s on a regular basis. Of course it depends on the type of file and whether you are reading or writing. Writing is slower (20MB/s). This is on a gigabit network.
The Drobo FS is really quite nice and simple and it’s fast.