To match the PPI of the 15", it would need to have a resolution of 2560 x 1440, same as the 27" iMac and Cinema display. The HD 4000 will probably struggle if the 650M GT struggles with 2880 x 1800 - it needs a GPU that is 70% of the performance of the GT 650M.
That pretty much points directly at the 640M GT as it is almost exactly 70% of the performance. The 640M LE model is a possibility at 60% as it shaves off about 10W and still 2-3x faster than the HD 4000.
You can run a 27" display at 2560 x 1440 from an HD 3000 though:
The lag seen on the 15" retina might be down to the scaling process rather than the resolution.
I hope they don't go with a 128GB entry SSD. I think 4GB RAM is ok to hit the price point but 256GB SSD is a must. A 640M(LE) with 512MB-1GB memory would make for a very nice 13" MBP.
At $1599-1699, it has a good chance of killing the 13" Air. I think next year, they will drop the prices on the retina models and kill off the 13" Air, old 13" and 15" Pro together. I'd like to see them drop the 11" Air too. I think it's a compromise too far. Instead, just ship a 13" MBP with a non-retina TN display and IGP for $999 with the same chassis as the Pro and 128GB SSD. While the taper design does give the sense of thinness, I much prefer the Retina MBP design.