Yes, it is very difficult to estimate the tax gap. There is a lot of ongoing research in this area, both in Canada and abroad, and there is an awful lot of research that looks at the use of tax havens for both tax evasion purposes and tax avoidance purposes.
I'm not sure I would say that it's not a useful exercise to try to estimate the tax gap. Here in the U.S. there is an annual exercise by both academics and by the government to try to come up with a tax gap estimate.
The problem is that the bulk of the missing revenues, I think most researchers would agree, would be as a result of domestic underground economic activity, particularly non-compliance in Canada with the GST: I'm a roofer and I only accept cash—hypothetically, of course—so I don't pay my GST. That's where I think a lot of the tax gap comes from.
Nevertheless, there could be more attention paid to this issue by Finance here in Canada.