Due to a recent bug in the Gun Mettle update (which has since been patched), Specialized Killstreak and Professional Killstreak kits/fabricators can be found in MvM and applied to any item (including hats).
Since this patch, the only way to obtain such items is via the Steam community market, and they're very expensive.
I'm considering buying one, but I need to know beforehand: do the Professional Killstreak effects apply to the hats? e.g. If I have killed 10 people with a Killstreak shotgun and am wearing a Professional killstreak Gibus, will the Professional Killstreaker show (even though I have no Professional Killstreak weapons)?
Best Answer
From the wiki:
Killstreak items work by counting the kills made by the weapon in a single life - for the counter to be incremented, it must be the weapon that kills a player.
Since you cannot kill players with hats, and assists do not count to a killstreak (removing the possibility of a hat (e.g. alienswarm parasite) counting kills), any killstreak hats will be useless other than their title, and to gain any killstreak effect (including those of the hat) you will need to already have a killstreak weapon logging your kills.
A killstreak hat will not give a sheen, but WILL give other killstreak-unique effects.