I'm having trouble understanding why a monoflow system would return hotter water. It might, but I don't understand why. Yes, it's possible some possibly minimally diluted hot water could leak through, but a lot of the water passes through each convecter, so cools off as it passes each. Keep in mind also that those efficiency numbers are based on 'normal' cycles where the boiler minimizes its startup and cool down losses. At best, on the design day coldest temp, if your load is 30K, and your boiler is 74K, the boiler would run less than half of the time. A mod-con would be running 100% of the time, and thus have no startup cooldown losses, so even if it wasn't condensing (which I think it would), it would end up more efficient. Now, cost-wise, I don't know how those two compare to purchase and install, but considering a boiler can last decades, and fuel prices are only likely to go up, every little bit helps. The break even point may not be good, you'd have to calculate that.
Even with an outside reset controller that adjusts the boiler temp on the high mass unit, that would likely cause it to short-cycle even more since it can't modulate the burner. It would make the house more comfortable by reducing the cycling of max heat by letting it use a more moderate temp in mild conditions, but to supply that moderate heat, it would be turning on and off the boiler more (much more!) than a mod-con. Cycles affect not only the efficiency but the longevity.