Not really noticed any issues with the standard leather even after downpours, although I tend to use a half tonneau. I would save your monies ...
+1, mine has been through torrential rain and snow without any problem with the standard leather.but there's no harm in going XT if it gives you peace of mind.
I use Gliptone waterproof leather conditioner a couple of times a year and after 28k miles it's all still in good shape.
I'd spend the money on a stage 1 exhaust - the single biggest improvement to the M3W you can make.
Also yes to the rear mudguard - which is easy to remove if needed to change the rear tyre.
