Hi. I like the dimensions of your HT. However, if you move your front seat to just 8 feet from the display, you will have a couple of issues, among which are:
1) You will restrict the placement of your front speakers. What if the optimal location is 4 feet from the front walls? That would put the speakers possibly too close to the listening positions. What if you want Wide and Height speakers? If the front seats are too close to the display, there may not be enough room.
2) 8 feet from an 85" (diagonal) display is close but doable. Unfortunately, if you decide to have a bigger screen in the future, you would be too close. Given the size of your room, you should really have a screen that is 133" diagonal minimum. Otherwise, those sitting at the back will see a very tiny screen and you waste the size of your room.
Good luck!