Hi Darren,
60% is pretty good for data plane - having an iBGP peering will not increase this much if at all - you'll still only be loading the best route into FIB - whether that be the one from the EBGP peer, or iBGP neighbour, but by all means give the clustering a shot.
With regards to VRRP sorry I should have been more clear - I was referring to VRRP on the "inside", not on the carrier side - the next-hop your clients would use to get to the primary SRX.
In the situation you describe, I'd have a reth interface on the inside (with a member interface from each chassis node), but on the carrier side, I'd have a dedicated interface to each SP (eg: ge-0/0/0 and ge-9/0/0)
The default-policy statement is as close to packet-mode as you can go, so yes - it's just like ANY/ANY on all *zones*.
You will still have to place your interfaces (reths or whatever) into a zone (can all be in the same zone if you wish) or else they will not forward traffic.
You can also override this behaviour via policy if you wish - policies are consulted first, and if there are none, or no matches the default-action is checked next.