I am going with 19" wheels both front and rear, so for rear I want a 30 sidewall and for front, a 35 sidewall, it looks like I have some really good options for tires in 19". I will lower the car all that I can, so I don't think it's gonna be any problem with not filling the rear quarters out.
It is not easy to find the right tire combinations, damn it takes a lot of time.. Like you, I have also spend a lot of time to find the right tires and are still not sure what I should get! The tires is also really expensive when you get over 300 wide..
A 315 is also some serious wide rubber, I'm sure it is going to look so cool!