Front Derailleur 10T Capacity (XT FD-M8025)

I don't know the severity because I rarely encounter recent mountain FDs and hacky setups for them even less, but what will happen is the contact it makes with the chain when shifting off of the small ring will be suboptimal. That kind of problem will vary with what cog the chain is on in back, i.e. better in the lower gears. There can also be problems with the chain wanting to drag on the cage in some small/small combinations.

You'll probably get a better performing setup by running a wider cassette and cheating the RD total capacity. Shifting will be largely unaffected and you'll only be down the ability to use some small/small combos, which will be compromised anyway if you do what you propose.