The 13.8v of the 7355 will get the batteries to accept x amps, but 14.4v will get the batteries to accept way more amps, so that is what the 'boost mode' does on the three stagers.
It just runs at a higher voltage. The converter can't do any more amps than rated for, but the batteries will take more of what's there if the voltage is higher.
In addition, resistance of wiring and connections between the converter and battery reduces the amps getting to the battery. Going to 14.4 instead of 13.8 gets more amps past that resistance (not an accurate scientific description, but gives the idea)
If the 7355 is miles from the battery as in many RVs on long thin wire, then replacing it in place with a three-stage converter won't do much good because of the wiring still the same.
So you get deck-mount converters that can be placed close to the batts, or if that is not possible, then you at least need a converter in the old place that has a manual 'boost' button (mounted or remote) so you can force it to go 14.4 when it otherwise would not on that long wire, seeing that high R.
