Right, I got the numbers crossed up but as stated in my post, the Merlin has gear reduction to produce torque. Its true about the prop tips going super sonic but they design to a balance between that point and diameter for clearance and number of blades to absorb the power to turn it into thrust.
Also, gear reduction ratios vary by Merlin/Griffon model type and application.
What a couple guys have done for RC has been about 0.5:1 ratio or greater to really reduce the prop RPM and increase torque. Their final RPM at the prop is roughly 3000-4000 and they can handle the full scale model diameter with only a 55cc DLE or similar small engine.
Thats the final point to this; standard model engines cannot produce sufficient torque to spin true scale props. If they could the prop RPM has to be slower and the pitch higher than normal to achieve the same flight speed.