I know what you mean about fat tyres but the p38 had 255s on and that was just fine in the wet.. you can't tell me that had better handling than a more modern car but the l322 does have a lot more 'toys', you don't need to hit the brake pedal for the various stability control thingy bobs to start doing their thing. Maybe i should turn it off and see how/if it changes anything what could go wrong...Only if your braking through the water. Checking your brakes is a good idea or maybe don't go out in the rain. Being honest there are a few models of land-rover that should not go out in the rain.
I think the wider the tyre the more the impact onto the waters surface giving a bigger reaction.