Excuse me - MacPro way behind? Way behind what, how? Please tell me. I'm using a mid-2010 MacPro with 24GB of RAM, 24 effective core processing capacity, 2.63 GHz clock-speed and this thing is blazing fast and rock solid even processing 1 gigabyte files. There are PCs that can do this too, if you want all the performance-degrading baggage that comes with virus, spyware and malware protection you need for those systems (but by the way we may soon be needing for Mac too, as the cyber-criminals have now developed a toolkit for hacking into OSX).
No - from what I've read and observed of my own work sending images to the 4900 in both the 8-bit and 16-bit print options, one doesn't really need a 16-bit print path for a conventional RGB to inkjet workflow, because the heavy-lifting where 16-bit is important happens at the image preparation phase.