Antony
This may help in what you are looking for, but not a performance chart for any specific GA aircraft.
http://www.gasco.org.uk/upload/docs/...afety16-18.pdf
GASCo's Performance Calculator a.k.a. "Avoiding the Far Hedge"
On the subject of the 50' clearance limit - could be something to do with trees which are quite common near some airfields.