You're worrying about the wrong item(s).
The panel has it's operating current, which doesn't change. The only item that changes is the peripherals and any other powered device attached to the panel. The worst offenders are XOVR's, low standby but high draw for the outputs.
Your figuring on the loads incorrectly, standby is of no consequence, and in alarm condition, the panel will draw briefly off the battery to make up the current load, affecting backup battery time.
If you're running the system that close to the edge, you're asking for problems, since your alarm current would probably peak above 600mA easily.
Spend $30 for a supply, xfrmr and battery, it can be a dumb supply, and loadshed your piezos and strobe, since if those go down, it won't affect the rest of the system, and in actuality it will increase your no-ac standby time as a nice side effect.