My MB is the ASUS P6X58D-E. As far as I know the main issue is that the Marvell SATA 3 controller is just not nearly as fast as the Intel SATA 3 controllers that are built into MBs now. Some have said they can cause lockups though.
Yeah I've installed the OS then switched to AHCI afterward too tatty, but it takes a registry tweak to do it properly. That was not necessary in this case however because the BIOS was already set to AHCI for SATA storage type before installing the OS.
Device Manager is not a thorough way of knowing if your SATA storage setting is adjusted to AHCI properly. You need to look at the msahci registry value for it and it has to be set to 0. My Device Manager does show an AHCI driver like yours, but only one instance.
Since making this thread, someone on the Plextor forum got back to me and said to DL and launch the AS SSD bench tool to see how it designates the drive. On the Marvell SATA 3 port it shows as SCSI. On the Intel SATA 2 port it shows as ATA.
This makes me wonder if the Marvell controller actually acts as a SCSI adapter of sorts. Anyways, I've recently compared speeds again, this time at 4k to see which has better sysem performance. I get same read speed on SATA 2 & 3, but write speed is 70 MB/s on SATA 2 and only 59 MB/s on SATA 3.
So for now I'm running it on the SATA 2 port. If that Marvell SATA 3 controller IS acting as a SCSI adapter, I'm hoping this way there will also be less lockups in games.