I own a 2000 xk8 I purchased in 2003...it has a aluminum thermostat housing. The car sat for 5 years due to illness ...i have been restoring....the car is now up and running great....should I assume that before I purchased the car the first owner had the plastic one replaced... Or did some xk8's come new with aluminum housing.....i ask because I have noticed that xk8 owners are very, very detailed oriented....therefore I am wondering out loud if the original owner may have also replaced the tensioners....lol. As I write this I am laughing to myself..... So did the xk8 ever come with original aluminum housing...
I am doing most work myself... thanks to "TO THE GARAGE" youtube video's

 

Short answer, No, all plastic.
Bad news, only one way to know, if no documentation, take the valve cover off and have a look.
It does sound like someone was doing the right things, hope you are lucky.
Well, you already are, you own one of these beautiful automobiles.

 

Searched info on Thermostat and the opening range diligently. Appears this spec. is for S/C engines. The *S/C engines have a cooler range thermostat. Found this in an older thread post by Mod GGG. the following is copied from that thread. FYI: From JTIS, the design temperatures are; the thermostat starts to open at 80°C (176°F) to 84°C (183°F) and is fully open at 96°C (205°F). Perhaps the N/A engines have a warmer opening range, the **205 F. fully open degree seems ok.
*Rock Auto Catalogue show a separate stat than opens at lower temp. for a S/C engine.
**N/A stat has range from 187 F. to 205 F.
**My Max open temp is recorded from readings on Torque Pro.

 

disregard..