I can only make general observations from experience with SAN's ..

are your luns dedicated spindles - if not then you will have contention.

are all your trays/luns on the same fibre speed

are the number of spindles for each lun identical

I don't know how your san configures it's cache, but I'd say check it's config is the same - in general terms read cache tends to slow performance and write cache improve performance.

I've never found SAN engineers generally very helpful - there seems to be an inbuilt arrogance that performance should not be questioned !