Originally Posted by Sixty Fiver

The shifter is most relevant as it tells the derailleur how far to move.

The rd has to be able to handle the cog range and chain wrap for a specific drive set up and the cassette and shifter need to match up in regard to speeds.

You can usually swap derailleurs without issue... with SRAM and Shimano they have different geometry and spring rates that are optimized to work best with their matching shifters... the new 10 speed Shimano XT has a shifter with a different actuation rate than all other Shimano mtb parts.

Your solution is pretty simple... almost any long cage Shimano mtb derailleur will solve your issues and let you use a wider cassette, you will of course need to run a new chain that will be longer to handle the increased capacity and mate up nicely with the new cassette.