Looking at the spec's for this motor shows that it's rated for 6 cells (22.2v) which is what it would normally run in a 500 size heli. Also, in a 500 size heli you would normally run 425-430mm blades. By using only 14v and 600mm blades you are overloading the motor/esc. No wonder the rpm decreases as you add pitch. You need to run more voltage (22v) and less load (425mm blades) otherwise you're going to smoke the motor/esc. Also, watch your pinion size. Too large a pinion will also up the current draw. You should get a wattmeter which you can plug in between whatever power supply you're using and the esc. This will tell you what your amp/watt draw is.

Ed