exFAT....will I have a need for iPhoto?

iPhoto isn't really meant to replace an app like Lightroom. Aperture is the real Lightroom competitor in that sense. The only thing you lose access to is grabbing stuff straight from Photo Stream if you have an iOS device. Which may not be something you care about. Plus iPhoto will not play all that nicely with whatever app you use in Windows.

I'd probably format the drive as exFAT for the time being. It's the best option for sharing the drive between OSes.

Drivers exist that allow OS X to read/write NTFS as well. Paragon, MacFuse, some other ones I can't recall...

I haven't used them enough to speak of their reliability or usability in regards to iPhoto (iPhoto might do some tricks with extended attributes and HFS+), but they're out there if you haven't explored it as an option.

If you're using LR there's no need to use iPhoto. You're correct, iPhoto cannot use exFat for it's library.

other things to consider:
iPhoto stores photos in it's own library file (called a package) which can't be shared with another program (like LR) unless you duplicate your photos into the two systems (as you mentioned, keeping some on the SSD).

Time Machine will not work on a NAS drive and if it does, it's not officially supported (so if it happens to work but then one day everything dies, well, you're on your own). The only "NAS" drive that Time Machine will (officially) work with is Time Capsule.

Why dual boot? Why not use Parallels or Fusion to run Windows in? Or better yet, why run windows at all? You bought the best MacBook Pro out there and you're still debating which OS to use? On a rMBP I don't think there's any comparison, and on Mac hardware I'd think you would want to run OSX as your primary OS and only run windows when you absolutely had to. You will definitely get the best Mac experience (and retina) using OSX.

All of my external drives are formatted in exFat for easy compatibility between multiple OSes with the exception of my backup drive which is HFS+ to support Time Machine. While others have mentioned that you can get NTFS drivers for Mac (or even HFS+ drivers for windows) the thought of installing drivers back and forth for compatibility does not appeal to me at all when there's an alternative that just works. Of course, some things (like iPhoto) will not work with it but pretty much everything else does. I even keep my iTunes movies on exFat.