That's not bat at all for a small stove. I've had a small stove for the past four years, but not as efficient as what you're talking about. Finally went out and got a medium size household stove that will hold a fire for 14 hours. It does take up more space but during the winter it's well worth it to wake up to a warm area after a good nights sleep.
Then again, since the weather is warmer I'm thinking of wheeling into a shed for the summer instead of hauling it around. I'm wanting a stove pipe going out one of the old high up blinkers, on the rear of the bus, that don't seem to have a purpose anymore.
And that's what I like about my minimalist setup. Upgrades are where I find them. Maybe I'll find one of those efficient stoves sometime. I really like that stove that has the oven in the bottom, but I don't like to carry expensive stuff in the bus.