SteamOS is a Linux distro ; chances are, yes, you can. It is however probable that the kernel will need some time to be optimized for that setup and would miss a few drivers (the touch ID one mainly). There will probably be some patches coming in the next few months, and we'll see. I don't think Asus will be forthcoming with patches...
15% is bunk, unless you’re opening a bunch of browser windows, with a video in one of them, and using a dual-core CPU maybe. On something like the Ally, with an 8-core CPU, Windows’ background tasks probably amount to 5% of one CPU core. Even on a Steam Deck, it wouldn’t be 15%.
Places (including Tom’s) have run benchmarks under Windows and SteamOS with a Steam Deck. Performance is usually very close or favors Windows. There’s a reason for that. Yes, binary translation can hurt SteamOS performance. But the biggest delta I’ve seen in favor of SteamOS was around 10% (for a game with a native Linux port). And most of the time it’s more like 2~4% at most. 15% is a massive outlier. That’s in line with the bigger overhead of Windows: 2-4%.
It seems you misunderstood me : Windows overhead is 2-4% when your CPU runs at 3-4 GHz; when it runs slower (like, say, 800MHz because it's out of juice) then you're in the 15% range (because telemetry needs its CPU cycles, and the antivirus, and the game bar, and the search index, and process isolation, and secure boot, and... Yeah, whatever it is that has Windows using 2.5 GB of RAM after a clean install, and Windows 11 running 10% slower than Windows 10 on a default install) at the very least - all stuff that Linux doesn't have. Add 8 cores running doing that stuff, and you have a winner - I have yet to see core parking actually working dynamically in Windows.
Remeber that the main advantage Zen3 had over Zen2 was larger CCX, but since Van Gogh had a single CCX its IPC is close to Zen3's; also, it also uses DDR5 RAM, meaning it's not that much slower than Zen4 on that front... So, guess what runs better for games ? A quad core with 10 watts and little overhead, or a 8 cores with 10 watts and lots of overhead? The other big difference between these cores is cache size, but cache requires power.
All the tests I've seen show that when it comes to low power gaming the Steam deck gives the Ally a run for its money and runs longer - up to 5 hours on a single charge, where the Ally has trouble going past 2.5 hours.
And sticky buttons, lousy joysticks, no trackpad, and no repairability - having tasted Asus' aftersales service on their ROG hardware, I hold no hope on that matter.