SwordGuy - It is a bit of a grey area to be honest; the T&Cs do not specifically state under different scenarios. It simply states that you need to move to a standard residential mortgage if you leave the company. The percentage you would need to pay would be based on the current valuation of the property at the time, e.g. if you bought it for $2m (this is HKD by the way, not USD sadly!) but it was worth $3m when you left the company, on a standard 70% LTV mortgage, they would lend you up to $2.1m. So in that case, there'd be no issue; you could just switch without any further deposit required. Obviously if property prices went the other way and you were in negative equity, it wouldn't be so pretty.
How far they would actually go to enforce this in practice is difficult to tell; I suspect if you were made redundant you might have a case to argue, but if you got fired or left of your own accord, maybe not so much!
How long til I could retire... I'm still working on figuring that one out! While our current spending is low, as an expat in HK we are faced with the prospect of school fees here in the coming years for 3 kids, which may change our spending levels dramatically if we decide to stay!
Waltworks - Yields are pretty reasonable I think, so does need a bit more investigation. I do have some investments in the stock market currently which I'm holding on to, though it's true that most of our excess is going towards the mortgage at the moment to try and get it out the way.