Hello Eric, thanks for the question.
Sorry to say, but there is no easy answer to this question. After reading your post I only have more questions.
First off, how much space do you actually need? This question may answer what raid type to use (example being that in Raid 6 you lose 2 drives to parity over 1 in Raid 5, or losing 1/2 of your drives in a Raid 1/0 configuration). If you can't afford to lose more than x drives, then certain RAID types can be eliminated from a possible solution.
Next, you need to figure out if you bandwidth hopes are realistic. To aid in this I point you to page 71 in the EMC Best Practices Guide (Flare 29.0 version). The charts there show what total throughput and bandwidth values were seen under testing (specific IO load and tests of course). If you IO size will be on the low side, then large bandwidth numbers may or may not happen. Being sequential with certainly help with this.
Are you backing up many luns/drives? At the same time? Is there a window where this needs to get done? Is there a write window to this array and then a read window or are they overlapped? These questions in my opinion will decide how many groups to go with. If you need to back up multiple luns/drives at the same time, then what may be sequential IO to the front end will turn into somewhat random IO on the drives if the devices are on large raid groups (drive heads keep cycling between two or more spots on the drives because of the front end load. This drives up service times and drives down the amount of throughput/bandwidth possible.) If you have multiple devices to back up at the same time, then more groups of smaller disk counts will be needed.
Another thing to consider is rebuild times. Eventually you will see drive failures, but if you go with very large groups the rebuild times can be huge depending on the rebuild rate you choose. Raid 6 will help with fault tolerance, but the rebuild impact on performance could be substantial.
I hope these questions/topics help in designing a solution for you. The performance rabbit hole can get very deep quickly depending on how many things you are trying to control.