Just to add a conclusion to this:
I configured a host trying both options for iSCSI seperately, different then same subnets, both returning long boot and rescan of HBA times. Our exisiting two hosts had been running fine for some time but they hadn't been rebooted in a while but couldn't recall long boot times. I tried a rescan of the HBAs on the working hosts and they too know had this issue! Eventually found this article http://kb.vmware.com/kb/1016106 referring to RDMs and MSCS which was set up in our environment earlier this month. I perenially reservered the LUNS used in the MSCS cluster on all hosts as per the article and everything is now good.
Thanks again for your help.