The benefit of straight water alone is going to be dependent on the AFR that you would need to use to suppress knock.
My assumption is the psi's previously listed are within the efficiency range of the turbos.
Is there a fueling and timing combination that would permit you to run 20 psi on pump gas at all without knock? I know it wouldn't be near the 1100 hp you currently get with race fuel but could you do it for instance with a rich fuel setting and retarded timing?
If you can, then you will be able to tune back up as if you were using race gas.
First tune the car with its rich fuel settings and retarded timing to permit a knock free (albeit not powerful) tune.
Then add the water injection and start leaning the fueling. Your power will start increasing. Once you are happy with the AFR power and water injection is suppressing the knock that would otherwise occur - then you can start looking for MBT. It will likely be about a degree more advance than you were getting with race fuel.
Time to tune wise - you are probably in good shape. Clearly you have enough experience with your setup that you have a good feel where to go run to run. To start with think of when the water injection is engaged you have just switched from pump gas to race gas. That is in general terms it will be slightly different.
The nitrous switch is probably binary and not pulse width. If it is pulse width and can be mapped then you could use it. If it is binary on/off then you could use it to drive an fia2 system which would match your water flow approximately relative to your fuel flow. This will not be as optimum as mapping water to your torque/cylinder pressure curves but it will be a big help in moving you towards your power goals while suppressing knock.
Based on another post which brad responded to - you may even be able to do it with only one shurflo pump instead of two, brad could answer that better. With Aquamist pumps you would definitely need two and some accumulators to achieve your desired flow. Either way plan on a few jets.
The f-body below has twin turbos with no intercooler. It is using two Aquamist pumps and by my best guess of the routing 6 jets.
Richard could tell you more about this customer's results but it was impressive. He previously had a huge front mount setup with no water injection.