Why doesn't that answer work for you? Each business is different so 3sec load times may be a max for some where 8sec is acceptable for others. In the end "pageviews, queries, visitors" is going to be specific to the application being run and they all add up to load time metrics.
Shared will work till you start receiving enough requests per second that the page load times become "unacceptable". Where unacceptable is somewhere over 6sec, when visitors start hitting the back button to google. How many requests/sec the shared host can handle before load times become unacceptable is dependent on the shared servers hardware and configuration. You are at their mercy.
I personally don't see why more people aren't using VPS solutions like linode and slicehost. At $20 a month you have root access. What that means is you can run tests on drive IO performance, cpu performance, etc. Additionally, you are more secure than whatever chroot environment a shared hosting provider places your business critical application in. You can recompile php to make use of opcode caching with APC, etc. You can decrease load times by optimizing server performance instead of upgrading to higher tier shared hosting.
Basically, from a math standpoint it would seem to me that shared is only a good value to those who wish to do little if any administrative work. The value is with VPS where solid, secure, flexible VPS solutions start at $20 a month. Perhaps I'm biased but this is serverfault.com so I'm in like company.