Does Training Grounds reduce the cost of activated abilities that cost X?
Ie if I declare X to be 8, would Training Grounds then reduce the cost so that I pay 6?
The activated ability will cost (2) less and 'x' just means you choose the cost. Whatever it winds up being, pay (2) less than that. It was even used as an example on the MtG website.
Metathran Aerostat: This one's extra cool. Let's say you announce X as 2. Even though you only wind up paying , you can still pop any two-drop onto the battlefield. Now let's say you have two Training Grounds on the battlefield, and another Metathran Aerostat in your hand. Announce X as 4 and pay only , and you can have the two Ships trade places. If you have some positive effect out there as well (such as Pandemonium) you can turn that into a win condition.