I recommend reviewing the EMC CLARiiON Best Practices for Performance and Availability, FLARE Rev. 30.0's Rebuild and Stripe Size sections.  This document is available on Powerlink.  However, consider performance and availability of small versus large RAID groups.  From a performance standpoint, the stripe size of the larger RAID group will abet large I/Os.  Backup likes the 8+1 and 8+2 stripe.  You also have a greater number of IOPs in that larger RAID group.  From an availabity standpoint, an 8+1 can take a long time to rebuild, especially if its near capacity.  You also need to consider the availability 'failure domain' aspect.  An 8+1 has a higher likelihood of a drive failure than an 4+1. Also there can be a lot more in-use capacity with a 8+1 than a 4+1; consider the effect of a double failure.   See the EMC CLARiiON Storage System Fundamentals document for a discussion of Mean Time Between Failure (MTBF) and calculating the likelihood of a RAID group drive failure based on a 100K hour MTBF.  This document is available on Powerlink.
We are having a discussion if we should go with raid5(4+1) or raid5(8+1). Other than space saving advantages with (8+1) what else do we gain and I know there is going to be a performance impact if we go with (8+1) but what else are we going to loose.
RAID5(4+1) vs RAID5(8+1)