Should I exchange the SX50 for something better in low light and action mode??????

It really depends what you want.  this is a good lightweight carry around camera for me when birding.  My SLR is a 70/300 f4/5.6 lens so I am not losing anything really.   A 600 2.8 lens would require a mortgage to buy and a cart to carry and goodness knows what a 1200 2.8 lens would look like.  I would have thought a 1200 lens 6.3 lens is as fast as you are going to find out there.