Once you flash to ND8, you cannot downgrade back to the MK3 bootloader or the MK3 modem. You can install a MK3 Rom or a MK3 nandroid Rom backup. If you are going to need to Odin back to stock, you must use ND8.
I am on ND8 and I do not have any connectivity issues, very good battery and no GPS issues. It all seems to be centered around where you live, what Sprint I'd doing in your area and what other mods you have done (i.e. greenify, comadose, etc.) Hope this helps.
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