7½ x 17 ET37 is perfect offset for a Sapphire. I don't understand what the problem is? Someone has obviously fitted longer wheel studs at some point in it's life to accomodate the 8 x 17 ET30 wheels. It is not a problem to cut these down to the required length or swap them for some standard size studs.

Offset is how far the wheel sits under the arch. The smaller the number, the further they stick out. I would imagine that you must have had arch clearance issues with the ET30s, as they would normally be ET35 in that width (and would still require some arch trimming with certain size tyres)?