I ran a 225/275 18 combo and wished I had gone to 245s all around. For me, I like a car to rotate and with the 225/275 combo, it just didn't happen. I won't say if 245s are faster, but they do feel more fun. 245s all around will help reduce understeer, but realize that the Boxster left the factory desgined to understeer - especially with the 205 stock fronts
(plus, understeer is safer and limits lawsuits)...
If you are committed to a "stock suspension" then you will
always chew up the outside edges of the fronts. R comps need at least 2-5 degrees of negative camber and the stock set up only allows about -1.8 at most (unless your car was in a collision
)
So, an advantage to 245s all around would be to spread that wear evenly around the car by rotating wheel positions.. and then flip tires on the rims as well.