firstly, he asked about max tyre width, personally i wouldnt run it or reccomend it, but im answering his question. there isnt as much room up the front due to the shit amount of inner clearance, so ultimately the front will never be able to fit as much tyre underneath.
2ndly, +30 offset at the rear is to push up as close to the rear tie rod, in case u didnt realise, a 17x10 +30 will stick out further than ur 17x9 +20, it will also stick in further, giving u more room for a larger tyre. a 17x9 +20 is not utilising a lot of room inside the guard, in fact theres about 20mm worth of unused space there.