Although MSD coil packs are very good, if your current OEM coils are not faulty, this is not money well spent as it won't provide you with any improvements. Get a cam over a set of coils it will provide you much more bang for the buck.
Or since you are getting a cat back system, another option over the cam would be a set of Headers (long tube) and high flow cats. Again much more bang for the buck over a set of coils.
As for the Octane level, until you get your tune, moving to higher octane than recommended in the book serves absolutely nothing. In some cases people have actually seen a lower MPG as with more octane the Gas is harder to ignite and burn appropriately forcing the engine to compensate with more useless fuel...
You will always get better performance (throttle response and MPG) with the octane level that s recommended for the tune your engine runs.
Also if you have a tablet or smartphone, you may think about spending a few $$ and get yourself a bluetooth OBDII interface to monitor engine performance and condition. They are cheap and provide good information to help troubleshoot some conditions, and also just for the fun of monitoring the engine parameters.
As for the plugs, don't get fancy. The HEMI's run best with copper core plugs like the OEM champion 439 or similar plugs from other manufacturer. Most here prefer the stock OEM plugs, but I personally run copper core NGK's without issues. If you tune your ride though, you may consider a set of hotter running plugs to better withstand the additional heat generated by the tune and higher octane gasoline.
Here is some good explanation on the Spark Plugs Heat range :
Spark Plugs Heat Range explained
The adjacent pages can also help in making an engine diagnostic based on spark plug condition. I often take my plugs out, just for good measure and see if they can reveal an issue with how the engine is running.
Hope this helps your decision making in your build.
Have fun and stay dirty.
