I'd say I'm somewhat experienced dealing with headhunters having worked in one for a short period of time and having dealt with them countless times.
First off I agree with most who say their interests are not aligned with the applicants for obvious reasons. Their client is the hiring company not the applicant, even though in a way, they represent the applicant. They'd bargain for a higher salary if and only if you're the only candidate or if their client absolutely loves you. Normally a reputable company within an industry, either a big international firm or fast growing boutique, would not give you a salary that is lower than your current one. If the hiring company is offering you 27k while you're earning north of 30k, this is probably not a good company. Having said that you currently working in Canada makes it complicated because they have to adjust for cost of living.
On the flip side, it's the sad truth that both the headhunter and the hiring company will treat you as a local, because you look like a local, speak the local language like a local (despite your fluent English, which by the way, is not that much of an advantage because many HKer speak fluent English), and probably have the same personality/mentality as a local. So my advice/comment is, unless you're desperately looking to move back to HK immediately, don't take the offer. Tell them your expected salary (you should do some research as to how much this level pays) and leave it at that (IMHO, 27k is too low but 40k is a bit high) If this doesn't work out, you can apply for others. By them finding you, it probably means you're not looking actively anyways. While it's harder to apply for jobs overseas, with a unique set of skills and experience, you can apply to jobs yourself and probably find one that suits you.