@Saller See this post from noodler:
More info...
When I'm working on getting the "bear trap" of the boot shell to close more completely over the ZipFit liner, here is the process. Right after inserting the hot liner into the shell (with my foot in it), I will first close the instep buckle of the shell while being very careful to ensure that the overlap is seated correctly. Then do the ankle buckle followed by the top buckle (the toe buckle is basically meaningless for the liner fitting). Flex the boot quite a few times and then go back to the instep buckle and tighten it. Then the ankle and top buckles again. Keep cycling through shell flexing and buckling tightening as long as you can handle it. When you can't handle the tightness any longer, pop open all the buckles and relax, but don't take your foot out of the shell. Your goal is to get the liner to break-in and form to your feet and the shell. This takes time and patience to do it correctly, but the end result will be fantastic.
If the Corsa is already fitting quite easily into your shells, there's a good chance that it will need more OMFit compound along the way. I'm willing to bet the Gara is the better match for your foot and shell.
While you're waiting to ski run through this protocol a few times when you have the time. For heating the liner put your oven as low as it'll go (zipfits can't get over 150F and most oven get down to around 180F), place the liners on a wood cutting board, then leave the over door cracked. Give them maybe 5-8 minutes in there, if you have an infrared thermometer then you can take the guess work out, if not go buy one because they're fun
. You aren't trying to roast them you just need to get them very warm to the touch, but not painful to hold.
I ski a pair of WorldCup Sidewinders that I have 20+ days on (they're basically a gara but all leather and they have a plastic reinforcement panel on the medial side that partially extends over the navicular area, so even stiffer and more volume) that I originally bought for a pair of Salomon XMaxs that had packed out. I couldn't even get the top two buckles closed on my first attempt. After repeating the above process a few times though I successfully closed the buckles and could get them as tight as the second catch but they were non adjustable. I now have the exact same shells as you, which actually feel roomier in the instep than the Salomons, and I can close them comfortably snug on the 3rd or 4th catch with the ladders in the default middle position, although they still are slightly tight over that instep/crook of the ankle area at that setting. Usually when I'm skiing though I don't notice any discomfort, mostly just noticeable when stepping out at the end of day.