I know we're getting a little off topic, but ...
I'm not seeing this with my Navien CH-180 boiler. I have it set right now at a max firing rate of 30% for heating (I did this last season after seeing the same efficiency curves you posted above). On a really cold morning I will temporarily bump up my supply temp from 130F to 140 - 145F for more rapid recovery and it has no problem rapidly increasing supply temperature. However, I'm talking about going from 68F to 70F t-stat setting - hardly recovery from a deep setback.
The degree of oversizing is going to be pretty ridiculous at max fire of a CH180. It takes 150K-in at max fire, so 30% of that would be 45K. Even at 86% efficiency that would be 38,700 BTU/hr, which is more than the heat load of my house @ 0F.
If you have a 15F delta-T across the boiler, you won't be getting more than 86% efficiency out of it on those recovery ramps, since the return water at 145F out would be 130F, which is above the condensing range. If it keeps up over night with 130F output water at a 15F delta-T the return water would be 115F, and you'd be getting better than 90% efficiency out of instead of 86%. That's a 5% difference in heat output with fuel use. So if the setback was saving you 5% in fuel due to the lower heat loss out of the house at lower indoor temps, you ate up all the savings during the recovery ramp.
But it's the RETURN water temp that's critical, not the output temp. At 145F out with a 40F delta-T it's going to be doing AWESOME, ( but that isn't a typical radiation setup.) At 130F with a 5F delta-T it'll barely hit 90%.
If the radiation is so limited that it needs 150F water to just keep up at very cold temps, the setback approach would save money, since it won't be condensing anyway. That's why you get more heating efficiency out of a mod-con when design condition heat can be delivered with 140F water, which takes quite a bit more radiation. But that's also why outdoor reset is critical, so that you can reap the condensing efficiency advantage whenever conditions allow.
Every house and every system is different- there is no really simple way to model this. But know this: If you have to bump into non-condensing temperature ranges to recover from setback when it would have kept up at a condensing temperature, you've saved effectively nothing (and maybe even lost something) by using the setback strategy.
The mini-split setback efficiency issue has other issues, and they are more pronounced than with mod-cons. At sub +5F outdoor temps you're not going to do much better than a COP of 2 no matter what speed it's running, but at full speed you're looking at 1.6-1.8. At +15F cruising along at part load the COP can be 3-3.5, but at full speed it barely breaks 2. This is way more than a 10% hit in operating efficiency! In an overnight setback situation where you are trying to get the place back up to temp before breakfast, your recovery ramp occurs during the coldest part of the day, and the absolute efficiency is at it's worst. It's hard to come up with reasonable algorithms to adjust the ramps for reasonable recoveries from deep setbacks. Mini-splits also have very little output compared to a boiler, and at lower outdoor temps the capacity is even lower, which makes for very long recovery periods during cold weather. Here again, most of the time limited setbacks during the overnight hours is going to use less power.
The Fujitsu AOU-xxRLS2-H series cold climate mini-splits have an occupancy sensor auto-setback feature that can be programmed on or off, and is marketed as an energy savings feature. But unless they did something really sophisticated to limit modulation levels during the recovery ramp (doubtful) it would increase energy use. Limiting the modulation of the thing during the recovery ramp guarantees lower comfort, since it's triggered by the occupancy sensor. When the room temp is 10F lower than the setpoint you want that heat to come on as fast as possible when you're actually there, not wait an hour or three for it to bump back up. An occupancy sensor isn't like a programmable thermostat, that can anticipate when the place has to be up to temperature, and possibly work some control magic based on indoor & outdoor temps, and the time at which the house needs to be at the setpoint.