Don't work at an airfield (anymore). but sometimes get calls in TC of "we can only hold for 20 mins then we will have to divert...
I personally like the 'heads up' as it means that we can already start putting a plan in action - warn the GS so that he can be ready to spring into action etc.
However, As LEGAL TENDER says above, I would not treat an aircraft that said 'if we go around we will call a PAN/MAYDAY' any different than a normal aircraft, partly because as DFC so correctly states, this could be classed as just another play on the old 'fuel emergency' call - if the controller allows his or herself to be suckered in.
Sterile runways are par for the course when a declared emergency is inbound - there are different local rules used for the distance ahead an aircraft needs to be to be allowed to land ahead of the emergency (in part answer to legal tenders rhetoric question).
Where does it stop... windy day, 5 in the pattern, number one breaks off due to windshear. Does he/she state then that if they go around 5 more times they will need to declare an emergecy? Does that mean they get bumped ahead of other aircraft?
Unless an emergency is called, then I would treat you as a normal aircraft.
The MATS quote from Vortex Issues [QUOTE]...If the pilot declines to do so, the controller may, if he thinks it appropriate, carry out the necessary actions as if the pilot had declared an emergency.../QUOTE]to me means more that the AFS etc will be readied and not that other aircraft will be broken off... however it is down to individual interpretation.
The long and the short of it is, until you declare an emergency, you are in my book, Ops normal. Giving us a heads up to aid planning is, however, always good and will mean that you will get a better (swifter/smoother) response if you do need to declare.