Here are some things to chew over:
A link to H6K Data
http://www.combinedfleet.com/ijna/h6k.htm
Model H6K4 - 127 Units produced (most representitive of this AC)
184 kt at 4,000 m Maximum speed
120 kt at 4,000 m Cruising speed
Max speed fits very well if we consider cfg figures in Knots.
So at 184Knots the H6K can patrol 6,800 Km^2 per hour in SH4.
And if this was at 120 Knots it would be only 4,400Km^2 per hour in SH4.
These figures assume 20Km wide search path which is again in CFG file data.
This AC has an endurance of approx 18Hrs.
Which means in essence the H6K can patrol 42,480Km^2 MORE on one tank of fuel than it should be able to.
Not only this but on an attack run where the AC is already at top speed upon initital approach to bombing its target the difference is:
Distance to your submarine IE: Plane spotted long distance 8km range.
Top speed: 5.6Km per minute 1 min 25 Seconds closure time.
Cruise speed: 3.7Km per minute 2 Min 10 Seconds closure time.
You should expect perhaps 20 seconds more time than we have as the enemy AC picks up speed from cruise upon attack.