THis is what I would do to satisfy all parties involved...
1) RTA / Police - get your rims engineered and that should deter any problems with them ... legally can go 1 inch wider than original ... so .. 5.5 + 1 = 6.6 inches
ps. check what the stock rims on the ST162 is .. are u sure it is 5.5 inches ??
2) Insurance - if 6 inch is all they will insure then that is that. Ask them if the rims are engineered will they insure ... otherwise change ur insurance company .. haha