There are two distinct limits imposed by rear mechs that you need to check before lowering gearing: a) max rear sprocket size the mech/hanger can handle, and b) wrap capacity which ensures the chain is still adequately tensioned on small front small back when the chain length is correct for large front large back.
According to
this, for a 6700 SS (i.e. the short cage version) you have no problem going to 28T regarding a). Depending on the frame/hanger (in particular the distance between the axle and the rear mech fixing bolt) it is very likely that you can go to 30T. In any case you can check by measuring the clearance you can see between the existing 25T cog and the top jockey wheel when you are on that gear - every 2mm allows an extra tooth - if you don't get enough clearance
adjusting the B screw may well give you enough.
Regarding b), based on the linked tech doc the 6700 SS has an official wrap capacity of 33T. If you put a 12-30 on that leaves 33 - (30 -12) or 15T for tooth differential between the largest and smallest rings on the chainset. So whether a 12-30 cassette breaches b) depends also on what chainset you have - if it is a standard double or compact you should be fine. If not it depends on how wide the difference is and how conservative Shimano's spec is in this instance.