I expect an 8 hour layover at Guangzhou airport - are there facilities to have a shower?

Sure. VIP Lounges are available and they are only for first class and business class ticket holders. There 6 lounges for First/Business Class passenger, they are on the third floor of West-I Bridge, third floor of West-II Bridge, third floor of West-III Bridge, third floor of East-II Bridge, third floor of East-III Bridge, third floor of East-I Bridge(For international First Class).

If with an economy ticket, you may apply for a stay permit, and walk to the Pullman Hotel, opposite the terminal building, to take a shower and have a good rest.