Spec taken from a retailers website:
Graupner Speed 600 Race 7.2v Our Price £15.64
Click to enlarge Product information A high-speed motor with integral cooling fan for racing use in high-speed model aircraft or model hydroplanes (boats). Suitable for up to 7 NC cells of 1.7 Ah capacity. Suitable propeller: SPEED PROP 15 x 13.5 cm. Specification Nominal voltage 7,2 V Operating voltage range 6 ... 8,4 V No-load rpm 22635 min?1 No-load current drain 2,4 A Current drain at max. efficiency 17,7 A Current drain when stalled 130,3 A Max. efficiency without gearbox 70 % Length of case, excl. shaft 57 mm Diameter 37,5 mm Free shaft length 6,5 mm Shaft diameter 3,17 mm Weight 235 g
Assuming this is the motor, the nominal voltage range is 6v to 8.4v, so with your 3 cell lipo at 11.1v, the voltage is too high and you risk overheating the motor.
If however, the motor was for example a 8.4v graupner bb turbo, the voltage range then increases to 14v. I tonight ran that motor on a 11.1v lipo, and the performance was way better (speed and runtime) than a 7.4v nimh pack, as the discharge rate of the lipo pack (ie, how fast it can give up the juice!) is way better than a nimh pack.
Unfortunately it would appear your retailer has provided duff info