and presumably the same with a large .psd or .psb file, with all its masks and layers?
I'm not sure what you mean. But whatever you mean, try it and see. I just did the following as a test.
1. Took a new, RAW file into Photoshop. Made a bunch of layer adjustments. Saved that as a psd file.
2. Imported the psd file into LR. Made a number of further adjustments in LR.
3. invoked "Edit in Photoshop". You have 3 choices.
4 First choice is "Edit a copy with LR Adjustments". The adjusted LR file opened in Photoshop as a single, background layer. Made a number of other adjustment layer changes. Invoked Save. It saved back into LR as a psd file with identical appearance as it just had in Photoshop. Now have 2 files in LR.
5. Second choice is Edit a Copy (without LR adjustments). Your file as you originally imported it into LR opens in PS. It shows each of the adjustment layers you originally did in PS. Add more adjustment layers. Save. It saves back to LR as a new psd image. So now you have two files in LR.
6. Third choice is Edit Orig (without LR adjustments). Your original psd file opens in PS, showing each adjustment layer. Make further adjustment layer changes. Save. It saves the newly adjust PS image back into LR as a psd. It overwrites/replaces the image that had been in LR. You have one file in LR.
I personally always use the first choice, because I want to preserve the improvements I've made in LR when I take a file to PS for further work.
Brad