Labeled boot size is somewhat flexible. One brand's 27.5 might be bigger or smaller than another brand's. Then there's width. And instep volume. And cuff diameter.
You want the boot to fix as an exoskeleton. Snug and comfortable like a lobster's shell. A fitter can make a boot bigger, but has few techniques for making it smaller. If the toe really is tight, it can be ground out or heated and punched out. Expect something like that. We all have lumps & bumps on our feet, and we do not want the boot so big that it accommodates all those spots out-of-the-box. Rule of thumb--if the boot is comfy in the shop, it's too big. The liner will stretch and mold to fit your foot; this isn't really packing out. A packed-out liner is worn out and due for replacement.
A shell fit should show something like 1/2" to 5/8" of room from your heel to the bare shell with your toes lightly touching the end of the shell, liner out of course. (If you boot pros have a different measure, let us know.) Forget finger fit. One finger, two fingers, whose fingers? In the shell, centered, spread the boot open and look at the gap on the sides. 1 to 3 mm of space on the sides is good. More than that is too much.
I ski with a guy with feet like pancakes. Wide and flat. He needs a low volume boot heated and stretched to fit him. I have feet like carpenter's pencils. Long, flat, skinny & skinny ankles. I need a low volume boot plus a thick insole plus a tongue shim (and a punch out for the knob at my left 5th metatarsal). Neither of us gets a good fit otherwise--too much volume above the foot. I ski with a woman with high insteps. She needs a boot with volume there. This is why there are so many brands and models of boots. Keep trying until you find the one that fits as snugly as possible without discomfort.
We haven't even talked about flex yet. (1) there is no consistent industry standard about flex numbers. One brand's 100 might equal another brand's 90 and a different brand's 110. (2) Opinions about what flex rating to get vary about as much as opinions about next week's weather. I like a stiff boot. I have boots labeled 130 that ski like they may be 140. I'm not a racer. I like the quick response when I put some input for the ski into the boot. And, I like the stiffness to brace against when I'm off balance and need something to lever against to get re-centered. Others like boots with more flex. Good luck. You're sailing into barely charted waters.
Wintersteiger (the ski tuning equipment company) has a 3D laser foot scanner which "provides a three-dimensional image of the customer's feet in seconds. It also supplies a wealth of measurement data such as instep height, ankle circumference, foot length and width, thereby helping sports retailers to recommend suitable insoles and ski boots."
https://www.wintersteiger.com/us/Group/News/Latest-News/238-WINTERSTEIGER-wins-ISPO-Award
If they also have scans of the interior of every make & model of ski boots to match to the scans of the feet...that would really be something interesting.
A nice, comfy, bigger boot will quickly become sloppy and then become uncomfortable in its own right. 
