Re: Internal routing
In general routers are better at routing than firewalls, of course, but it all depends on the goals. If backbone/core router does routing between branches/network segments then firewall becomes pure perimeter firewall, guarding Internet access for those segments but doing nothing to protect each one of them from another.
600E is a beast, nothing to compare with 100D but a lot depends on how you are gonna use it - services he is going to run vs traffic volume passing through it. Give each branch fiber of 100/100, enable URL filtering with SSL inspection, proxy mode, identity based rules, VPNs etc. and even 600e may strain. Look at the existing patterns and extrapolate to the new 600E capabilities. If 600E is gonna do the same load 100D does today + routing, then it will eat it without sweating.