we have an PS6510e which appears hitting the I/O limit of the array - Is it possible to upgrade it by replacing some of the 7.2k SAS HDDs with SSDs?
No. Dell/EQL doesn't support changing drive types on existing arrays. You would also have to reset the array to accomplish this change anyways. Since only RAID6-Accelerated would work with the combination of SSD and SAS.
Have you opened a support case to determine if you are in fact hitting the limit on that array? What makes you believe you are hitting the limit?
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