I am sorry, but as a personal athletic instructor there are better (and safer) ways to lose weight (what weight fat or muscle or water)?
It is true that weight can be lost by wearing sweat suits, HOWEVER, Almost all of this weight is water weight sweated out by the body. Although exercising in warm environments enhances BMR, over heating can lead to excessive exertion, exhaustion, and fainting as well as hypoxia believe it or not! This is a dangerous idea, although I am sure under armour does not retain heat as much as perhaps you expect it to.