The taxon cynthia was revised by Peigler and Naumann in 2003.
The Hong Kong species is now referred to as Samia wangi (distribution is from Taiwan through N.Vietnam to easternmost Xizang, Sichuan, far south of Shaanxi to southern Zhejiang).
It is conceivable that Samia watsoni might occur in HK as this taxon has similar distribution, though slightly less southerly. The nearest confirmed records in Guangdong are from Shikengkong, Ruyuan and Yunwushan, Zaoqing.
Peigler, R.S. & Naumann, S. (2003). A Revision of the Silkmoth Genus Samia. University of the Incarnate Word, San Antonio, Texas. 230 pp, 10 maps, 228 figs.
If anyone is interested in meeting Ric Peigler, he will be visiting Hong Kong in late June (22-26) and staying at KARC - I may be able to arrange a moth evening at KARC during his visit.