I am in the business & most of my work is heating control upgrades & gas & oil boiler replacements. Saying that, I think SEAI is slightly exaggerating their figures or not fully explaining their publications.
Firstly, they work their figures from the most basic system & each point added gives a saving. For example adding a timeclock would add say 5%. I don't know a boiler system apart from solid fuel that does not already have a timeclock. Other additions are zone valves, thermostats, TRV's, etc. & each measure adds a saving.
High efficiency gas & oil boilers offer very differing savings.
Whilst a condensing boiler has a return temperature <20C lower than the flow, the boiler is in its most efficient state. This will deliver a saving of 15% over a standard efficiency boiler. If the boiler is over-sized, the system return will rise & eventually take the boiler out of condensing mode. At this stage, the boiler will only achieve a saving of approx 5% over a standard efficiency boiler.
This is for both oil & gas.
Where the gas boiler now wins hands down over an oil boiler & delivers far greater than 15% savings is its ability to modulate. A HE gas boiler will monitor its return temperature, turn down the flame to maintain a flatline demand & operate for longer but using a very small amount of gas. Think of it like boiling a pot of water on a gas ring. Full flame to bring it to boil, then simmer it just below boiling point to maintain the desired temperature but only on low flame, rather than full flame, let it cool a bit, then full flame, and so on.
Added with zone controls a gas boiler will behave exactly to suit the demand required of it at that particular moment, using only the fuel required.
This is where the future lies with system efficiency. Obviously insulation & heat losses come into play but I am talking about the heating system.