I've been looking for the same data than you for a long time and have had no feedback.
There is a post in the forum, where the propulsive performance of the ROV is analized, but Thrust-Torque vs RPM is not included.
I tried to get in contact with Graupner in order to get some technical information, but I've had no answer.
Searching for similar props I found Prop-shop (http://www.prop-shop.co.uk/). Asking them for some information, they answered me, confirming that they use model standard props belonging to Gawn, Wagenningen and KCA-ducted series.
From their information and by comparison, I've concluded that the Graupner 2308 family belongs to the standard Gawn-Burril propeller series, hence a theoretical-statistical study can be performed from the standarized polynomial expresions.
The standard polynomials have been obtained from real channel and sea trials, hence, are the most accurate way for performance calculations.
In order to be able for using the propulsive performance curves posted by OpenRov headquarters in the forum, for propeller data extrapolation, some motor data, that are missed, are a requirement.
Ro= Motor electrical resistance
Io = Idle current
PD: Dont hersitate asking for help.