I was in a similar position as you where my previous boots were my 'measured' size that felt snug with no slop or heel lift, just some wiggle room in the toes. When I downsized to my current boots, I was lucky all I needed was to heat mold the liners to make the fit 'work'.
I put 'work' in quotes because everything's great when I'm in the act of skiing; my feet feel locked in with practically zero wiggle room and feel like I can feel the shell around my feet better than before, not to mention there's a noticeable increase in response. The moment I stop skiing though, that feeling of 360 contact with the shell almost always turns in to pain within a minute or two. I've put about 50 days on the liners (intuition), so I don't expect the liners to pack out and relieve the pain anytime soon.
I've never raced, just ski off piste as much as I can and while I appreciate the increase in performance from downsizing, the pain has me considering going back to my measured size in the future. I never felt like my skiing actually suffered from the comfier fit of being in my measured size.
Just sharing my 0.02 as my experience could very well be a part of the can of worms from downsizing.
Oh and Phil made a great point about fit and downsizing not being just about the length, so keep that in mind too.