re: 'The advantage of using the the XR springs is that they are muck longer than the stock spring, allowing you to do away with the 4 inch steel spacer and using that space for springs.'
The 'longer' part makes them 'softer' than you might think. They are a lot more forgiving on the small stuff than a .40 of the 470mm variety.
Still, likely a bit much for 150# of rider-and-gear. Than again, following the FRP rec. idea, they may work just fine. ;)
re: the 5.0 shocker
A bit longer pullrod set will tone that down some. The devol set is only something like 1.5mm longer, so lowers the seat a bit less than 1/2".