Speaking from the position of someone who wants to see the best produced from any system, especially a MC system, and has encountered this scenario dozens of times, IMO if space is an issue for the rears, then maybe rears aren't the best option for you at this point. Besides, your be supprised of the result by adding a center speaker as opposed to only 2.
How much space are you refering to..ball park figure?
Depending on desired location, it may not be practical. However, its possible to achieve adding rears with not much space at all. It all depends on how much space is available.
A compromise of car audio speakers as rear is not the best way to go. If your not gonna use them as the front, then why use them for your rears?
As for the center channel, its always best to use a speaker as close to the fronts as possibe. Another S8R would be ideal.
If you want to bring out the best in your MC system, then the same precision used to build your 2CH system should be use to build your MC system.
Shoot me an email whenever you have some time.