I believe I can understand your reasoning, but I'm not sure it is right..... 
  
The 60 trim and 50 trim have the same overall diameter, with the only difference being the size of the inducer a little bit more than a 1/4 inch. 
  
I believe that the turbine will have a greater effect on boost onset and spool than the compressor trim. 
  
If you are adamant about the 50 trim (and I'm not saying you're wrong), I don't think that a Stage 5 turbine will gain you anything down lower and will definitely help up top. 
  
Just using the same Stage 3 turbine with a 50 trim compressor would give you better onset and spool than what you have now, but the Stage 5 turbine trim will go the other way....