SATA drives, such as the 3TB WD unit being discussed here, don't use SCSI CDBs. They use SATA FISs, with 48-bit sector addressing. So there is no "2TB limit" for LBA addressing. Ditto for the SATA controllers (Eg. AHCI) as well.

Some OSs might internally use 32-bit numbers to hold LBAs, in which case those OSs could suffer from a 2TB "barrier".

There are still BIOS boot issues and partition table concerns, but those wouldn't be unique to "AHCI mode" versus "non-AHCI mode".

Cheers