Hi,
In my case it is like you already said it.
We have a lot of customers with a shared storage. When it comes to plan a upgrade, we take a look at the hardware which is already in use. In most of our cases it is really easy to find a new storage to replace it with the old one.
I can give you an easy example:
A customer uses 4 esx hosts with a netapp 2040 and a additonal shelf, in summary with 36 x 300 SAS 15k 3,5" in an FC-environment and has 50 VMs running. Let's face it, if the customer has any performance issues, he would let you know about it and asks how to get rid of it. Then you have to analyse and find a way to get better performance. But let's pretend, the customer is quite satisfied with his environment. When it comes to replace the storage, and there is a normal growth of the infrastructure I would recommend a FAS2240 and at least one additional shelf with summary 48 x 600 SAS 10k 2,5". If the budget is big enough, you can add an second shelf with 24 disks.
I don't know in which sizes you think, but in a small environment (like my example) is this a way to go for me. We did this a lot of times and never had a problem with storage decisions. But I have to say, we know our customers for years and have a good overview of thier IT.