It's existing since some time meanwhile, yes.. She's more looking like a male topless, not even a proper A-cup, as this is the no-clipping sub-body mesh. Quality of life for the designers as it's not clipping with armor, bad for those nude mod users out there. ;)
Rather keep the clothes on in that game. Would need an entire new mesh to fix that issue. So yes, the cup size she's wearing in armors and co is late A to B cup, as there hasn't been any push ups back then nor the need to for societie's needs. :P
The preview images (which are bad) don't really do it justice. Tools and instructions are in the download, it's pretty straightforward. There's also a skin mod in the link (uses resorep) but other nude skins can be used if this one isn't to your taste.
I'd suggest using the olympic outfit (unlocked by progressing through the game). Where the basic body is built to fit beneath armor, the olympic isn't. It makes Kassandra fuller at the front and improves her shape in the back, and has an oiled version available.