Cheers for your help guys, I have talked to a guy running 87+ 8" 944 rims as apparently these have a compatible 52.3mm offset. They don't project inwards at all so the extra inch is added to the outer side of the wheel.
What I have concluded so far (please feel free to correct me if I'm wrong):
The dished ones are from 944's from years 86 and earlier, as far as I know these have a 23.3mm offset that stick out 55mm more than my current rims... but also have 29mm more clearance on the inside.
The 81-84 928 (unfortunately mine is a 79 model) have a longer stud with a factory 21mm spacer, removing the spacer and using open lug nuts will mean the earlier 'lipped' wheels will just fit under the guards
Good spotting Riber, that car is identical to how I would like my 928 to look, it's even silver! Even though the author wasn't able to help me with offset etc they did confirm that the car was definitely running dished 944 rims. I think the car must have been a 81-84 model as mentioned above.
Another option is to get 3 piece autostrada monza's like these!
http://farm5.static.flickr.com/4100/4912712901_bb9a4016bd.jpg
Modifying a set of dished 944 rims would be ideal for both handling and perfect fit, I'm going to take my car down to these guys: extensivewheelservices.com.au who recon they might be able to widen the existing wheels without me having to do any modifications to guards etc. Good point though Danglerb, I hadn't even thought about getting tyres yet!
Thanks again for your help!