Replying to Boots significantly smaller than shoe size?

So this is my first year skiing after switching from snowboarding, and I went to a bootfitter so I wouldn't fuck myself with shit boots. The bootfitter told me I should get a size 27.5 full tilt and I tried them on and they fit, although they were tight, but my shoe size is somewhere around a 12 which should correlate to like a 30 boot size. Has anyone else had a boot recommended that is a lot smaller than their shoes size would suggest? Also, the boots I got (2013 Tom Wallisch Full Tilts) are size 27.5, and they are pretty tight, not too tight to where I can't wear them, but tight enough that I can't see myself riding a whole day with them. How much of a difference will heat molding play in opening up the boot to make them more fitting/comfortable?