Glad you mention Magento but shame it's on hold!
I'm working on this myself and aim to adopt your vB approach. You are right, when they put something in the cart or start comparing items, pages should start to be rendered dynamically. Or with such "dynamic holes" indeed. But if LSWS can't do that (yet), I'm still going to try full page caching.
It will be good for Google to begin with, as Google won't add anything to cart or start comparing items. It will also be good for first-time visitors. On their first landing, they have nothing in cart or comparison anyway, so may as well impress them with a speedy website. As they come to know the site, use it more, trust the brand etc. speed can be sacrificed a little bit for extra functionality.
For the cart, you can even come up with fancy was like putting cart content quantities in a cookie of an external URL which then through external JS renders that text with cart contents. But until that, I'm happy for full page caching for new visitors and Google.
Just need to work out a Magento module which keeps a cookie that tells LSWS to show a cache page or not. My plans are coming along nicely, aim to have something working mid-March.