You made some mistakes(mixing up units).. the lowest possible temp is -273.15C or -459F, or 0 kelvin. (absolute zero). Increase in flame temp when combusted with air will be approx +10.4C.. (0.04 * 2483K) + (0.96 * 2223K) - 2223K = +10.4K.
+10.4K/(2223K-290K(or 323C)) = 0.53%(17C) to 0.55%(50C) increase in efficiency to help offset 4% H2's 2.71% reduction in energy content. Since most applications are direct thermal usage(water/hot air/etc) no additional losses will be incurred.
As for losing ~2.2% of heat energy, in exchange for cleaner burning fuel. It's a tossup.