I personally believe that when starting heroics you should bump up your mana regen and then cut back as you feel comfortable with rather than running on fumes unnecessarily. I went from 1600->2300 Spirit and took the 2 set from LFR (I still had a 378 gloves so it averaged out as an equal ilvl/stats exchange) for now.
Depending on your tactic on Zon'ozz (1 camp or 2) it can be very mana intensive in both phases which should lean you towards more regen, I noticed issues on Yor'sahj heroic but that was without the Mana Void trick so that is a pending issue. Spine/Ultraxion seemed rough enough on normal to warrant higher regen too.. but obviously how your guild plays and your style of healing will impact this issue more for you personally than anyone else can guide you.
Are you talking about 10 man or 25 man Zon'ozz here?
I'm off-spec healing in our 10m raid and also healed Zon'ozz 10HC a few days ago using a 8/2/31 mana spec with <1200 spirit, two int trinkets and 4t12. Mana is tight but I never felt I didn't have the mana to keep the raid alive in our attempts and our kill; without dying I could sustain ~29-30k HPS for the whole 6 minutes.
Thus I'm definitely keeping my low spirit setup until there's a fight where I really feel mana starved before the end of the fight.