Don't worry, be happy. A tankless WILL be more efficient, only if you have 3 showers in the house that are likely to be running at the same time, or if you have fairly hard water it would it be way less than ideal. Rinnais are definitely quality goods- I wouldn't sweat it. The biggest operational issue with the R94LSI would be late-summer very low flow temperature regulation when the incoming water temps peak, since it's lowest fire is 15K, but that's as good or better than most- it'll be fine. Just be sure to clean the filter at least once/year (put it on the calendar).
If your unsure about the hardness your water supply have your water tested for hardness. If it's under 1 grain, forget about it- not a problem, between 1-5 you may have to descale it every few years. If it's over 10 you may have to descale it annually or more often to keep it up to snuff. (Installing isolating ball valves and taps for descaling ports at the same time that you install the HW heater isn't a bad idea in hard-water location installations. You need to be able to run a mild-acid rinse trough it- there are commercial descaling/deliming solutions are available, but dilute vinegar can work if you're too cheap.)
But if you have any sort of hydronic (pumped hot water) heating system, an indirect-fired HW tank running off the boiler would usually be a better (cheaper, lower maintenance, yet still high-output & high-efficiency) choice.