Hey Tucker,
Those above options both seem to give you a gable overhang, but there are other options as well.
You can extend your plates past the end wall posts by however much you need it (2'6"), then add another pair of rafters. It depends on you roof loads, but then you likely would need braces coming from the plate ends back towards the frame into the posts to support the roof load above. This option can work well, and does a good job solving the structural problem, but then you're left to deal with air sealing around any member that's passing from inside the frame to outside. I think you might also have to check with your local building code, as some states frown to interior structural members being exposed to the outside. However, even so, there seem to be options to getting over that hurdle.
2'6" is a lot to ask out of a SIP. As far as I know, SIPS cant overhang that much unsupported.
You can also look into roof brackets, which can support a big roof overhang, but then you're looking at the same air sealing problem as mentioned before. This time though, the brackets are purely exterior members.
A cross-hatched roof works well, which gives you an overhang by putting wide boards on edge and cantilevering them beyond the last gable. This, however, wouldnt work for your current roof system.
Hope that helps,
Dan
