Martin is right (lol ! ) , split rims are a 3 piece wheel , the centre has no discernable offset , the inners and outers are whatever size you require to set the wheel in your arch , in fact almost no-one uses the term "off-set " when talking about wheels , in Racing you ask the "Backspace " as this gives the hub mounting flange to the inner lip of the rim .
By the way wider than 9x 17 is very possible on a std arched RS 500 as i proved with my black car , ive gone for 12's on the new car tho and 290 tyres for grip !!