The interface throughput on a PIX 525 is about 340 mbps and the aggregate throuput on a 525 is about 370 mbps. The reason for this is mainly that the 525 only has a 32 bit 33 mhz PCI bus.
I view the 525 as a PIX that is able to connect to gigabit networks. If you need a true gigabit firewall, you will need to upgrade to the PIX 535.
Hope this helps.
Scott