Oh I could go mad on there!
I'm usually a 14 but in that style dress my waist isn't.
My wedding dress ( also a fixed style dress) I had to get a 20 and had to have it taken in on the top to get it to fit my waist.
They took it in on the shoulders which raised the waist a little. Where the waist had been in the right place and perfectly cofortable, raised up a bit didn't leave much wiggle room. I had to eat very carefully in the days leading up to my wedding. They also took it in under the armholes.
( I should try it on again and make sure I haven't put any weight on!)