I can't speak for the factory, but rigging over the rated HP is not good practice.
Our Islander is much bigger, and we purchased it new with a single 250HP OX66. After 465hrs we repowered to twin F150s. The Islander is rated at 500HP and the F150s are a good match. We have 880hrs on them since Nov '05.
I would recommend the Yamaha F150s for the 257. They are conservatively rated, and seem to test at about 162hp. If you want more, and have a good Suzuki support dealer, then the DF175s would be good as well.
I would not exceed the rated HP on the boat.
Happy New Year.
Brian