The semantic issue here ACHI or SATA or IDE for that matter, is one that probably has not been addressed correctly by the motherboard manuals, or for that matter, the BIOS software engineers.
Part of the problem resides, (as you referred to) is the nomenclature existing in these entities.
It's not entirely avoidable though, due to the current existence of 3 different Windows OS in use simultaneously. But the definition, description, and emphasis of drive mode has, in fact changed. My experience is with Intel, so I can only give that as my point of reference;
The Intel G956WHMK board I bought, (before I joined 10/'06) offered "ACHI"in BIOS, but with the annotation that this could only be implemented with Vista, and not with XP.
A Gigabyte G41 chipset board, give me "auto" as a mode, then when selected runs the HDDs in ACHI, or a least when system info utilities are used on the system, they list the "current transfer mode" as "SATA 300gbs.
The BIOS in a brand new H55 Gigabyte board only gives 2 options; "AHCI" stated directly or "IDE". No mess, no fuss, no anxiety, but no "we'll work it out for you "auto" setting either. Just click "ACHI" in BIOS with Win 7, and you're on your way.
All of this makes sense, since the designers are catering to either the prevalent ,or expected OS installation. IE; nobody involved with hardware should be terribly concerned about someone installing XP in an H55 board. If someone does, it's on them.
All this about the Esata drive not being recognized, is IMO moot. The OP states, the Esata drive is only recgonized when it's turned on >> before << boot. I'm standing firm with the opinion that that's to be expected, since if it's not turned on (at boot poll) then it technically doesn't exist. Since the system is likely not true hot swappable RAID, why would it appear after initialization. (In the case of the H55 board, the only concession that is made to XP, is that BIOS >> defaults << to IDE). Although in the next revision they might think better of that, but who knows).
I agree that it would be nice if it did happen, but I wouldn't be terribly concerned if it didn't.
When the BIOS is set for AHCI in my H55 board, it fires up, checks the memory, shows the SATA controller then boots into Windows. There's no longer the crawl of the other devices being polled, nothing, just a big glorious boot into Windows in about 20 seconds.