Mics and placement are really the next place to attack it, and I might be very tempted by a second hand RE20 (But then I really like the things).
Changing the interface is unlikely to be cost effective compared to changing the mic, but don't forget that kit you will only use occasionally is very cost effective to hire in as needed, so if you have a rap act in and they want a U87 with an Avalon pre, but this is a not a regular occurrence then hiring is a good option.
One nice point about second hand mics, is that unlike any other studio tech, they tend to keep their value, a used RE20 or 421 or 4093 or whatever will keep its value much better then a new computer.
Past a certain point I don't know many clients that actually care what kit I own, as long as it gets the job done in a timely and efficient manner, on which basis I would probably invest in a nice coffee machine for the control room before anything else......
The computer upgrade looks sane, but don't forget that there will be endless pain while you get the new machine up and running. You do have a robust backup system in place?
Going back to the basics, how is your headphone distribution (And available cans), and how are you sorted for things like DI, Reamp and such boxes.
Audiophiles use phono leads because they are unbalanced people!