thanks for info, I hear that Lightroom uses the graphics cards but am not certain if that applies to the cards in the MP.
Yes, Lightroom will use the graphics card on my retina imac but it doesn't seem to use it very effectively. I can't recall reading any reports / reviews stating that the MP's graphics cards make Lightroom fly.
( I'm no expert on this stuff: I just went through a similar buying decision a couple of months ago. In the past I've used Powermacs & a previous gen MP but this time went for the imac. The new MP just didn't convince me on a cost performance comparison. Maybe it would if I was working 12 hours a day on 80 megapixel files & editing 4K video. As ever YMMV ).