Yes these are mostly reasonable guesses. But if you configure the existing 13" base model, add in 8 GB RAM, 128 GB SSD, it comes up to $1499. Throwing the premium on for the Retina Display then bumps it up another $1699--just below my estimate. There's three questions here: will it come with 8 GB RAM standard (like the 15"), will it have an i7 processor, and has the price of the display dropped enough to justify a smaller premium for the Retina Display?
My guess is that it will come with 8 GB RAM standard. Apple probably won't include a dedicated GPU in this computer. Having the additional RAM will ensure that this computer isn't prone to the sluggish animations that the 15" has on Intel graphics. I also believe it is likely to have an i7 for general performance reasons. This is supposed to be a high-end professional machine, slightly different than its entry level 13" without Retina Display.
Whether or not this display is come down in price, I have no idea. It may be easier/less expensive for Apple to make 13" displays than 15"; after all, they've been making high-resolution iPhone and iPad displays for a long time now.