RAID5 - 4+1 vs RAID5 - 8+1

I think this has been covered well, beyond some very wrong responses earlier in the thread.  But I wanted to provide some general rules for RAID 5/6.

• For performance - more total drives is better.  It is irrelevant what the RAID type is, or the group sizing.  27 drives has a higher theoretical performance limit than 25 drives.  There is no such thing as a read penalty, and write penalties are fixed by RAID type.  In this aspect, choosing 4+1 or 8+1 is not a performance question itself, unless we are talking about degraded or rebuild modes.
• For capacity on RAID 5/6 - the larger the RAID set, the more usable capacity available.  This means that a given 4+1 of X drives will have less usable capacity than a given 8+1 also using X drives.  However as you note in your example of 25 vs 27 drives, often there isn't a 1:1 comparison.  This can be driven by simply how many disks you have, and what they round to.  If you had exactly 40 600GB drives, you'd be looking at 40 in 4+1 (17152GB) or 36 in 8+1 (17152GB).  So while you would be using less drives, you wouldn't have anything to show for it unless you could leverage the remaining 4 disks in a meaningful way...not likely.  You do not want to use an 8+1 on 40 drives, even though your VNX would oblige and create it for you.
• For availability (protection factor) on RAID 5/6 - the larger the RAID set, the less protection available.  By this I mean, a larger RAID set has a larger fault domain.  It is more likely to experience a double drive failure on an 8+1 RAID set than a 4+1 RAID set, even though double drive failures themselves are relatively unlikely...especially on SAS or EFD drives.  Also, degraded mode and rebuilds are more taxing on an 8+1 than a 4+1, so you may see a larger performance hit on an 8+1 than a 4+1. 

There is an inverse relationship between 4+1 and 8+1 when it comes to capacity and availability.  While these are factual statements, the trade-off isn't immense.  By that I mean, if you choose 4+1 instead of 8+1, you won't be taking an enormous capacity hit.  And choosing 8+1 over 4+1 isn't going to automagically destroy your data (otherwise, why would it be a choice on an enterprise storage array? ). 

So while I've seen a lot of folks asking about this in terms of performance, it is not usually relevant with a 4+1 or 8+1 choice.  Instead you make a choice between capacity and availability.  For most enterprise organizations, capacity is relatively cheap up front so 4+1 is generally the recommendation and choice here.