Are you sure
that the NAS has a Gb NIC in it?
One reason that I ask: you said that your 360 has a Gb NIC in it. Last I checked, even the newest ones have but a Fast Ethernet (10/100) chip.
The transfer rate that you posted (17.6 M/s) is missing a character - was that 17.6Mb
(its)/s, or 17.6MB
If it was MB/s... 17.6*8 (converting Bytes to bits) = 140Mb/s. Faster than a 100Mb/s connection can do.
Losses would be (mainly) due to overhead at one end (or both ends) of the connection (most likely, it's your NAS being the bottleneck); possibly also affected by poor cabling, poor cable termination(s), or interference (e.g. routing cables alongside AC power lines, or next to flourescent lights.)
It's not your network topology - that's done right (your old layout, however, was very much 'not right.')
As Jip said, so long as the entire A-B chain (say, PC>switch>PC) is all Gb, you'll get Gb speeds - the traffic won't touch the router.
But, your transfer rate is still going to be highly dependent on the capabilities of the hardware on both ends. If the NAS can't process data all too quickly (and/or its HDD(s) don't have the best performance), transfer rates will suffer.