I would agree, before you start removing wheel / arch liners and stripping the tank out, just run it through with at least 50% mix of decent screen wash, keep it topped at 50%.
If you buy the 5 litre unmixed stuff, buy 2 or 3 bottles of it, then put half the first one into an empty 5L bottle (so you have 2 containers half full of neat wash) then top them both up with water.....and so on.
Then you'll always have 50% mix ready in the garage without having to guess by mixing it in the washer tank.
Pre-mixed washer stuff is a rip off, its more than neat to buy and is half water!!
E63 AMG here at last!