So I went ahead and installed the almost new Interstate MTP-91 battery on my 2006 997 C4S over the weekend. Even though this battery is a bit smaller (shorter but same width) than the stock battery by about 2 inches, I found out by accident when I removed the older stock battery that there are actually two battery securing plate bolt holes on the floor of the battery compartment and so was able to install it without any modifications....it fit in there perfectly!
It started without any issues and all systems were operational without any re-coding required!....that was a pretty simple project...FYI!
Cheers!
Alex