Hi-
All PPC G5's have a SATA-IO (SATA/150) compliant hard drive interface (Advanced Host Controller Interface) that supports transfer speeds up to 1.5 gb/s.
Drives that are the later SATA-IO (SATA/300 or SATA II) standard compliant, will have transfer rates of 3.0 gb/s, and with few exceptions, are backward compatible to the early SATA/150 standard, but will not achieve the later SATA/300 data transfer rate of 3.0 gb/s on an early SATA/150 compliant system.
The later SATA/300 compliant drives will operate within the limits of the G5 SATA/150 compliant controller, or have max. transfer rates of 1.5 gb/s.
http://www.serialata.org/namingguidelines.asp
So, the definitive answer is, No.
Drives that support 3.0 gb/s data transfer rates will not achieve greater than 1.5 gb/s in the PPC G5 due to the onboard SATA interface controller's being early SATA/150 compliant.
http://en.wikipedia.org/wiki/Serial_ATA