Tiger was built as a Battlecruiser with the lighter armour and building style to create the speed you talk about. Canada is built as a Battleship with the heavier armour and building style associated with battleships. It is easier to rebuild Canada to make it faster with better deck armour. To get Tiger to the same standard you need to rebuild the whole ship.
"Tiger" have 229-mm belt in citadel and 102-mm belt at the ends.
"Canada" have the same 229-mm belt in citadel and 150-to-102 at the ends. Only her turrets and barbettes was something like better armored
Generally, she wasn't a good battleship: she was more like an ultra-heavy-armed battlecruiser with limited speed. Her armor was sufficient against brazilian and argentinean 305-mm guns, but completely unsufficient for heavier artillery.
Using those 20/20 hindsight glasses, the RN was already considering 14" designs in 1930, which is the time you would be making the decision to keep or discard the Canada. More battleships being built with 14" guns would be nice if the shells could be used in both weapons.
Wouldn't it be simpler to just re-arm "Tiger"?