Wow 275 is huge, I was thinking 40 to 80 gallons.
I think you probably lost a lot of heat to the air, even small batch box stores are known to boil gallons of water rather quickly.
Peter van den Berg calculates the btus per hour of a 6" batch box to be about 77323 in the following thread:
That was 2016,he might have changed those calculations since then.
If you enclosed the heat exchanger, be it a coil or a tank, inside of a bell, you will increase the time the exhaust is in contact with it and that should improve heat transfer.
Most wood boilers seem to use a water jacketed firebox, but that's not good for rocket stoves.
A water jacketed barrel enclosing the riser would be great, but could require advanced fabrication.
I have successfully created a water jacketed feed tube with a steel inner cylinder , terra cotta outer cylinder with cement and silicon caulk in between.
Since tempatures never rise above 212F, regular silicon caulk and cement work fine.
Scaling up, a 55 gallon barrel could serve as an outer cylinder, a 30 gallon drum(or 40 gallon water heater) as the inner.
Stainless steel cooking gear is relatively cheap, heat resistant and has good heat transferring properties.
Steam table pans, giant mixing bowls, stock pots and bain maries all have potential as parts of a heat exchanger.