I have an 03 WRX and it would seem a logical move to upgrade from my 215/45 Z17's to a 225 section (as even my mates Golf TDI has 225/45/17's on it)!!

Will they work OK on the Standard 7" rim or will I need a set of STI Cast offs in their 7.5" width???

 

225's are fine on a 7" wide wheel, but you probably won't notice any difference. the overall diameter is slightly larger (will cause very small decrease in acceleration), wider tyre on same width rim will increase sidewall flex, total footprint on the road is increased and so will improve grip slightly.