I have the same specs, and get around 20-25 fps on low settings.
Try resetting the SMC: http://support.apple.com/kb/HT3964
And try this (I tried, and it works, but I am not sure about the performance boost): https://forum-en.gw2archive.eu/forum/support/mac/Config-File-Edits-for-Improved-Performance
I will keep on trying to improve performance soon, just bought this game but hope I can get it to run better.