If it is true that the /castsequence command has to wait for the server to respond before firing the next spell then we can semi-confirm this by doing something like

Shaman /castsequence LB,LB,LB this should see a lag issue and slow down dps

Mage /castsequence Ice lance, Ice lance, Ice lance Since they are all instant spells you should still be waiting on GCD even after the server tells your macro the spell completed. I would expect no slowdowns here.

Anyone got a mage and a shaman the can test on the same box?

(test both /castsequence runs vs just spam casting the spell with no macro. let us know how close each macro was to the dps of the spell etc.)