From what I've read the Mk II LR was a bit of a dog in the air as the slipper tank quite upset the balance of the aircraft. Quite understandably too I'd suggest.
I believe the PR Spits had leading edge fuel tanks replacing the cannon/mg of the fighter variants which bestowed a decent enough range for them and the various centreline slipper tanks were options, so the Mosquito type slippers would seem a logical progression.
Regards,
John