Well done, great loss again. Re tuna it is a tricky one, if you really want to up your exercise and that is obviously really important to you, specially at about only a stone from goal then you could try it, and see how you go.
I would be inclined to try the extra shake first just cos I think that is a safer option and they are basically protein drinks anyway.
I suppose as you are getting closer to the end of the journey you may get to a point where in order to keep hammering the gym you will need to up your intake and consider refeeding. Your body will let you know I guess.
Good luck with whatever you decide, keep us posted x