Boiler "hacking" - what's going on with my aquastat?

I'm trying to figure out how to correct and improve my Slant/Fin boiler.

It appears that after:

1. Performing the yearly maintenance service (under contract with our Oil Delivery company)
2. Replacing our old SuperStor SS-40 with a SuperStor Ultra SSU-45 (due to a slowly leaking tank in the original SS-40)
3. Switching the Taco SR503 Switching Relay to add priority back to Zone 1 (the circulator that heats the SuperStor)

The boiler began to exceed 30psi... I didn't notice this until I came down one morning and noticed water on the floor around the pressure relief valve discharge pipe.

After this I began to pay more attention and notice that the boiler would reach almost 30psi while running for a while, it would also reach just over 200 degrees Fahrenheit.

I had the plumber who replaced the SuperStor come back out, and he replaced the expansion tank...
...at the same time, I noticed the temperature reaching almost 210. I changed the Hi setting on the Honeywell Aquastst to 160 (from 180) This prevented the temperature from exceeding 200, but it would still get close to 200 (I will have to pay closer attention now).

Given the settings of the SR503 (Zone1 Priority for the SuperStor):

From furnace

From furnace

What should the Honeywell Aquastat be set to, and why would it not match the temperature gauge directly above it (e.g. when Hi was set to 180 and Differential to 10, the temp would still go above 200):

From furnace

Click below to go to the album page and see all of the pictures of my boiler, circulators, SuperStor, etc...:

furnace