I don't know where everyone is getting these backspaces from, but the main reason to have a frame notch is so that you can use a rim with 5" backspace or more.
I currently have 15x8.5 rims with a 5" backspace and I have plenty of room on the inside with a notch. On a 15x10 rim there is less bulge so you gain more clearance on the frame side with the same backspace but the main issue is clearance on the lip. The 10" rim moves the outside farther away compared to a 8" or 8.5" with the same backspace. If you want the rear height to sit close to stock then the tires will need to tuck in under the lip. You also have to worry if the rear is not centered perfectly and leave room for that or else fix it. If you go with less than 5" backspace, you'll need to make sure the car is high enough so it won't rub on the lip and having shocks you can make super stiff will help limit the travel to prevent rubbing the outer wheel well.
I was tired of the jacked up look and was able to lower the car in the rear with no issues once I went to the 5" backspace. With a 15x10 rim it would have been close on the outer lip on a 5" backspace.... a 5.5" backspace would have been ideal. See pics of my car in the thread
here. In these pics I have 275/60 MT drs on 15x8.5 rims with 5" backspace. I have a second set of the 15x8.5 rims with 28x10.5 slicks mounted but no pics with them on the car.