I have done some research and the maximum contrast of HP5 plus (independent of the chosen dev or concentration or agitation etc) is about 0.9. 
This isn't a big problem for me as I don't live in England where the sun seldom shines and most of my plans for this film will be outside with high SBR, and worst case scenarios will certainly require no more than N+2 development.
 Roger Hicks: "I don't think that Ilford can get much above about 0.9 for gamma infinity on HP5, and that was in D19R, but equally, many films of the 30s could go well above 1. A figure I seem to remember is 1.3 to 1.5, but I can't verify that without effort. " link.