I have a couple of things to add :
1. On the initial version of XBMC from January (the first release ever), I had no problems playing files up to 8GB on over wired windows SMB share (but ATV2 was on wireless N)
2. On the new version of XBMC, per the Feb 14th/15th release, and using the greenpois0n JB, I have the same issues as you do - buffereing for the same exact files on wired SMB. I tried installing a UPnP server(twesity / ps3 media server) and the buffering is awful! Exact same setup, but significant buffering on the same files that playerd relatively smooth before.
I am going to try a complete wired only test later in the evening to see if that makes it better and will post my observations then. Just so we are on the same page, streaming over wireless ATV2 is totally useless.. I dont believe it is XBMC's fault.. it is just the filesize / bitrate that is not sustainable on wireless N.
Hope this adds to the perspective / discussion.
And yet using the Feb 14 release on a greenpois0n JB I can stream an 11gig 1080p .mkv of Inception on Wireless N with no drops using PS3 Media Server...