I tried the older 13" retina, new 13" retina and 15" retina today in store.
The lag now is clearly down to the hardware, the old 13" retina still lags a lot even on Mavericks, the new 13" retina lag is a lot less, sites like the the verge are super smooth now which is great. However there was some lag during animation, for example, in about my mac when you switch between the hard drive tab and the display tab it lags. The 15" retina had no such tab that I could notice. The model I tried had the iris pro 5200.
It seems the 5100 is really good but just not enough still. I will wait a bit and also try my 13" but I have a feeling I will return if it's not good enough.