If the JP props are the same rating as other makes, the 'S' is a 1.2 to 1 diameter/pitch ratio whereas the 'X' is a 1.4 to 1.
This means that, for the same blade diam, the 'S' gives a LOT less thrust (and consequently less loading) than the 'X' series.
If the 40mm you tried was a 40X, then a lighter loading one is the 35X. - OR - you could try a 40S and work upwards until you find the most thrust without overloading the engine. They are available in 40, 45 or 50mm so, as has been said, its worth getting a few plastic ones to try. Graupner have very big front ends (don't know the technical term - I know its not leading edge) which tend to "grab" and need a size smaller than the old "Radioactive" ones. I don't know if Radioactive are still available but they may be the "JP" ones.
Plastic (strickly speaking - carbon reinforced plastic) will "give" ie straighten out under load, so if you need a bit more speed, find the plastic size and then get one size down in aluminium. Speed will increase but they cost more, though the range of sizes is far greater.
Danny