Installing new insert liner

Hey guys i am getting ready to install a new double wall flexible insulated liner to my existing older insert. I plan on doing the block off plate with roxul and doing the roxul below the top plate. The liner is 6 inch. It sounds like everybody has had good things to say after doing this. The chimney we have been using is masonry so I'm hoping my plan will push a lot more heat. I can only get the insert up to around 300 on the stove top which doesn't seem very high. What can I expect after I upgrade everything as far as performance. My chimney is 20 feet tall and has a 13 by 13 opening. Thanks

Thanks, I was hoping to get some first hand experience from people who have installed liners that didn't have one to know how much difference it made.