Windows on a Macbook Pro with integrated Intel Iris Pro only I've been wondering about this since the last refresh of the Macbook Pro range...I have a need to run some Windows-only software, and I've read in the past that many people complained about lower battery life running Windows under Bootcamp on their Macbooks. This turned out to be the result of the Discrete Graphics Card always enabled for Windows, with no way to only use the integrated GPU. 

I see now with the last refresh, that Apple have a Macbook Pro Retina with an Intel Iris Pro only option (no discrete GPU). Would it be reasonable to assume that I could get better battery life out of this new Macbook option under Bootcamp, than with previous Macbook Pro iterations? I'm certainly aware that Windows is nowhere nearly as optimised as OSX, and hence will not not be able to claim the same battery sipping status of it.

I work a lot on Magento and Drupal site development, and occasionally develop the odd nopCommerce (.NET based) project under Windows. I also make use of Adobe Photoshop CC for slicing up web designs handed over to me, and fool around in Sketchup every now and then. I don't play games on my laptop...purely work. So I believe that my graphics processing needs are very small.

Ok, I've gone off course a little, but getting back to the main question - Would it be reasonable to assume that I can achieve slightly better battery consumption by choosing the Retina Macbook Pro with Intel Iris Pro only option?
I have no sources other than personal experience to back up my claims, but I recommend the machine without the dGPU if you will be spending the majority of your time in Windows. 

I have seen the difference, and the dGPU kills the battery in Windows. My machine with an integrated GPU gets significantly better battery life under Windows than another machine with a dedicated GPU. In fact, the machine with the integrated chip gets almost equivalent battery life under both Windows and OS X.

You may want to stick with the dGPU if you need it for 3D gaming or media applications that perform better with a dedicated chip. 

In the end of the day, it is really up to you, but you will probably get better battery life without the dGPU. I have seen an hour and half difference in some cases.