Any Subaru ECU can read any level of boost you could possibly run assuming that you have a MAP sensor that can read that level of boost (and you logging the correct parameter - ex. MAP direct). However, with every 16-bit ECU, you can only target up to 39.45 psia. This is due to the way that target boost is stored. This is also the case for the Group N ECU.
The mode byte doesn't have anything to do with the IC switch. The mode byte simply allows you to disable/enable certain features such as anti-lag/LC (enabled by default) and per gear boost/wg comp (disabled by default). The IC switch is one of several requirements that must all be met in order for anti-lag/LC to be active at any given time (assuming they are enabled by the mode byte). The idea, I assume, is to give the rally driver the ability to control when these are active. With the USDM ECU, however, the input for the IC switch is used by something else which has a tendency to be active after warming up (meaning you have no control over it).
To enable FLKC, you will need to lower the FBKC disable threshold (which is why I added it specifically for this ECU).
With the 16-bit ECU, disabling of DTCs does nothing except disable the light on the dash. Any response to the DTC (if any), even when disabled, is still in effect. This could be the cause of some of the driveability issues.
Something that I haven't verified, but the Base Timing A/B switching appears to be based on whether the flash block is installed or not. I'm guessing this was designed to give the race team the ability to switch timing maps on the fly, but someone will have to verify this is practice to be sure.