have you performed any kind of baseline testing? It seems to me that you are getting ahead of yourself in your testing - going right to testing disk performance of VMS on an iSCSI device before you establish how well the disks perform on the Open-E host itself or how well the VM performs using local storage.

what is the performance of a single disk in your array? what's the performance of all your disks in the array when combined in a volume?
what is your baseline I/O performance on your ESX host? How do your VM's perform using local storage?

when you are performing this kind of integration, time and care must be taken to test each component of the final solution before bringing it together as the final solution.

it's a lot of work and effort and it's not for the faint of heart. this forum gets a lot of performance related questions. but performance is always relative. you can't expect to get something out of iSCSI that you can't get out of the disks natively .

that said, your question about delivering performance out of the box is an interesting one. I'd say "YES". You should be able to get that kind of performance out of the box when all the components are working well together. So I'd try and unbundle things and see what the individual performance is of my components.