@501Brandon I have the exact same boat, 2007 LXI Silver Edition, former Promo and upgraded from PP standard to Zero Off in June 2013. Great upgrade and then you don't need to get a new promo, you will basically have upgraded the boat to the 2011 LXI specs.
However if you already have PP StarGazer with Z-Box all dialed in, the difference for the driver and skier should be negligible. I've driven and skied behind several Z-Box Nautiques and they feel very similar to ZO where as PP regular now feels a little soft and spongy to me, not bad just not my favorite anymore.
If you have your heart set on a newer boat then I'd pass on the upgrade. If you really just want ZO then do the upgrade and keep the boat for 3 or 4 more years and enjoy the consistency and precision of the ZO pull.
Assume $3k for ZO, new ECM chip (plug and play) and installation. Doug Moss did my conversion.