Intel Iris vs HD Graphics – Is Intel Iris 1536 MB Better than Intel HD Graphics 4000?

According to the benchmark comparison tool here, the HD 4000 has between 2x and 6x the benchmark scores of the 9400M. (How relevant the scores are to your gaming experience is yet to be seen. Some of these are synthetic benchmarks, so they may not be wholly relevant.) The list is ranked with faster cards first, and the HD 4000 is #181, while the two 9400Ms are at #302 and #310.

tl;dr: It seems that the Intel HD 4000 should be better than the Nvidia GeForce 9400M.

Note: to be able to select the 9400M in the list, you need to deselect the "Still available (not archived)" box to show old cards.

I know this is not the answer you want to hear, but unfortunately there is no way to disable the discrete GPU and use the integrated graphics, when using Bootcamp.

This has been discussed (to death) on support forums and on the web generally.

I have thought myself in the past, that it would be useful to be able to turn off the discrete GPU just to improve battery life on the laptop. But Apple have not provided a driver capable of switching off the discrete GPU, so there is no way of doing it.

Sorry to be the bringer of bad news - who knows, maybe they will provide that functionality in the future?