You can get info on smog/pollution levels here
http://www.epd-asg.gov.hk/eindex.php
Tap Mun station might be a good approximation to Sai Kung.
Most of the pollution comes from the mainland these days, so, as you might see, pollution levels at all reporting stations are often rather similar no matter the distance from the centre of Hong Kong.
The Sai Kung area hiking trails are world class but try to pick an unpolluted day for the hiking otherwise you might be doing family lungs more harm than good.
Not supporting Taipei , but it must be said that the air in Taipei is gradually getting cleaner, while in Hong kong the opposite is the case.
GT