You may wish to check out http://wordpress.org/showcase/ and look for some large sites.
As for the mix and match approach you suggested, I tend to favour WordPress-only sites, mainly for the consistency in Look and Feel.
Traditional web sites are generally ideal for Caching in WordPress which can really reduce the performance issues that complex Pages can be present for huge visitor traffic levels.
In the professional I.T. world, shared hosting is generally a bad idea that goes beyond WordPress. Mainly, because we expect 24/7 99.9%+ uptime. A hacker attack on another of the thousand customers hosted on the same web server will cause response time issues (or worse) for your web site(s). Done right (web hosting company and you the I.T. guy), VPS (Virtual Private Server) is a better approach. Done wrong, it is much worse.
I am currently quite happy with the VPS of one of the recommended hosting firms listed here: http://wordpress.org/hosting/ I host 10 sites on a single 625MB VPS, each site having its own FileSpace (user (sub)account), and use no WordPress caching, even on a fairly high traffic site, all of which adds up to something approaching a worst case scenario. It costs me about $40 a month U.S. funds. I'm trading ease of use and increased margin for error (FileSpaces) for a few extra dollars per month.