I owned the original AM9 for a long time, and still have a pair of the later AM901s in semi-regular rotation; I’ve only ever tried on the AM902, but haven’t actually ridden in it. In short, the fit of the AM903 is still quite reminiscent of the earlier models, with some minor tweaks. Overall, the toe box and forefoot are moderately wide, tapering down to a more secure heel pocket. Compared to the AM901, the AM903 has a slightly more squared-off toe box with a little more room for the fifth toe (i.e., it doesn’t taper narrower as quickly) but is otherwise very similar in terms of fit; I’m running off memory here, but I recall the AM901 and AM902 feeling virtually identical.
At least on my feet (which, granted, feature pretty wide forefeet and comparatively narrow heels), the fit is outstanding. But I also think that the speed lace system that Shimano uses does a very nice job of snuggling things up evenly and effectively, so folks with narrower feet and/or lower insteps (mine are somewhat on the high side) shouldn’t necessarily write off the AM9. I find a little extra room in the toe box to be helpful on more gravity-oriented shoes for comfort when taking a lot of heavy impacts through the feet. Especially on those sorts of shoes, where I’m generally running my cleats more rearward for descending control and comfort, I don’t find a little extra room up front to be particularly detrimental to feeling like I’ve got a secure fit in the shoes. That’s not to say that I’ve got a whole bunch of extra room in the AM903 — they’re comfortably roomy, but reasonably so.
Compared to Shimano’s flagship Trail shoe, the ME7, the AM9 is substantially wider through the forefoot and toe box. In a size 44 for both, the Specialized 2FO DH is very slightly narrower, but significantly longer; the size 43 2FO DH is maybe a better size comparison to the AM903 in a 44 on the basis of length, and it’s appreciably narrower than the AM903 through the fore- and midfoot, but if anything the heel pocket in the AM903 is a touch more snug. The Ride Concepts Tallac and (especially) the Endura MT500 Burner are also significantly narrower overall. It’s also worth noting that Shimano’s European sizing runs somewhat on the small side — their size chart calls the EU 44 a US 9.7, which I’d say is accurate, and definitely the right size for me (typically a US 10), but I often fit better in an EU 43 or EU 43.5 from a lot of other brands.