I check thrust with a scale on the tail wheel of an airplane...
Years ago I built a large aluminum sliding test stand using linear ball bushings sliding on centerless ground hardened steel rods...There was a scale on one end and the engine mount on the other...
It was heavy, we used it to test 289cc twin race engines...
Using an airplane does the same thing...If used for comparison tests it makes no difference what the small offset from the engine centerline to the tail wheel is, all engines will have the same thrust vector...
I don't dispute the engineering and construction of the SAP, it's a museum piece..
My dispute is the amount of thrust with the 18-6 Vess prop...I think my G26 conversions and the DLE 30 are at least as strong, probably stronger...So I bought a SAP 30 to try...
Anyone that pays attention KNOWS the 3W 28 is the strongest in the size range...And the most expensive...A G26 or DLE30 for less than the SAP is a good deal for most people...
Some people have a need for the latest and greatest, go for it..But in the back of your mind know that a less expensive engine will outdo your/our new museum piece...