Balance 380 Rev 4 max WAN throughput?

I have an older Balance 380 hardware rev 4. Can anyone tell me what the maximum WAN throughput supported by this revision of the router is?

The online specs say that the router has a max throughput of 1000 Mbps, but it also says that the specs apply to the latest hardware revision. I’m pretty sure the latest hardware revision for the 380 is rev 6 and I’m fairly sure that revs 5 & 6 introduced faster hardware.

When I run a speedtest using a single WAN connection (cable) that is rated for 600 Mbps, it functions at that speed using a laptop plugged directly into the cable modem, I can’t get a reading higher than about 384 Mbps with the router in the mix. Can anyone say with certainty (or point me toward a spec sheet): is this the max throughput of the rev. 4? I need some proof to justify the purchase of a newer device.