I haven't seen any data on web dev shop profitability. But from what I am seeing, it really depends on how well you can sell. That is there are lots of small to medium businesses with managements that know very little and thus can be swayed by somebody who proposes a "solution" to whatever the management sees as being their problem or is convinced to believe their problem is.
In my experience, it is not about technology. If you can solve real problems in a particular industry and there are enough businesses in that industry then you have a chance. Of course, you might have to rise the bar to WebApps, etc.
It would be very hard to collect data. People, in general, do not disclose the P&L to outsiders. In fact, in many small businesses even the owners only know their P&L a year or so down the track when their accountant finally gets around to doing their books.