Doesn't sound like the matrix to me, it usually empties itself on the passenger side due to the design of the airbox. Plus if the matrix had gone and was chucking that much water you would defo know about it, you would have a very strong smell of coolant, steamy windows and also your coolant level would drop considerably.
Likely suspects are:
Missing grommet under the carpet
Plastic membranes behind the door cards, these must be completely watertight!
Rubber door seals and/or roof guttering
Sunroof, check the front & back drain channels are clear Fronts drain between the door & wing, rears drain out the boot area
Check under the plastic rain tray in the engine bay, either side there are drain holes. Make sure these are clear otherwise water builds up and works its way thru any grommets. A common one is thru the bonnet release cable grommet, either replace or just whack a load of grease round it. Also the fresh air intake on the passenger side can leak round the bottom.