It's an interesting situation you describe here and there is no easy answer. Like everything... "it depends" :-)  I'll start off by saying that this is certainly (as you already recognized) not a best practice. Concatenated Metas made up of different types of elements are never a great idea. Even if the first part had been RAID 5 on a 5 drive RG and the second part was RAID 5 on an 8 drive RG it isn't a good thing to do.  The biggest reason for this is not that performance will be bad. It's that the performance will be unpredictable. You won't know what you are going to get. If this is for a file server with low IO requirements likely no one will ever notice. If this is for a transactional database you could have situations where certain queries against the database run way better than other with no obvious reason. There are lots of other scenarios where this could be confusing.  What's happening when you concatenate the Meta together is that you basically take the first element and paste the second one on the end of it. If you start with a 100GB RAID 5 LUN and paste on a 100GB RAID 1/0 LUN you will continue to get the same performance you always did until you cross over the 100GB mark. Then you start using the RAID 1/0 LUN. Reads will come from whatever part of the Meta the data is already on. New writes will go to the RAID 1/0 LUN if you are past the 100GB capacity mark with your data. Writes over old data would go to the RAID 5 element. So you could potentially start seeing better performance an the newest things being read/written if the RAID 1/0 element offers better performance than the RAID 5 element.  As you can see it can get quite confusing to troubleshoot performance once you put disparate elements together. It isn't always a bad thing (in that your users may never notice or care) but it also won't ever be a "good" thing.  I hope this helps.
We are analysing a CX4-240 for a customer which was configured by some other partner.  We have observed that one the LUNs originally RAID 5 has been expanded using Meta LUN through concatenation with another RAID 10 LUN.  We know concatenation would not help with performance. We want to know what impact would a concatenation of RAID 10 LUN with RAID 5 LUN have? Would this also not reduce the performance?  Regards, Anuj
Meta LUN Creation