We don’t currently have the tools and not yet measured the flow rate or flow speed of our thrusters. However, you are right that with some estimates and math we can get to a reasonable rough theoretical value for these, at least under ideal conditions.
The T200 propeller has a 76.2 mm outer diameter, and a 40 mm diameter central hub. It also has a pitch of 22.5° at 75% of its radius, and spins at about 3075 RPM full throttle 12 V, and 3600 RPM 16 V. Using the pitch and and RPM approximation, this results in a theoretical maximum flow speed of 4.45 m/s at 16 V, and 3.80 m/s at 12 V. The propeller has an area of about 0.00330373 m^2. Multiplying these area by the flow speed results in a volume of about 0.014702 m^3/s (or 14.7 liters/s or 3.88 gallons/s) at 16 V, and 0.012554 m^3/s (or 12.5 liters/s or 3.32 gallons/s) at 12 V.
Note that the 55 W power number you mention is not the power generated by the thruster, rather the electrical consumption. The actual mechanical power will be lower due to the efficiency of the thruster.
Bear in mind these speeds and flow rates are rough estimates based on some math and not true measurements, I would expect the real number to be lower. However, they should be reasonably accurate for estimation purposes. I hope this helps!