I don't know how relevant merging in Photoshop is as a benchmark for Lightroom.
The way I use LR on my rMBP is to operate on smart previews. The full-res raw files are on an external drive, which I only leave connected for importing and exporting. All other operations (organising with the Library module, tweaking the with Developer, even exporting to the web, since smart previews exceed the quality requirements for this) are handled with Smart Previews. With this process I don't have to worry about space or having an external drive plugged in at all times I am using LR, and operations are fast.
So with this flow the importance processing capability of the MBP largely comes down to the number of images and frequency that you import and export.
As a photographer, I wouldn't dream of going back to a non-retina MBP.