I use a Black Coat of Plates sometimes, which is about one pound(?) lighter than your armour, with 6 athletics and no problems. 21/18 is a very good build, and so is 18/21, but I wouldn't go under 6 PS, and, to be honest, with heavy armour, you really don't benefit from high athletics as much as with lighter armour. The build I've had the most fun with as a 2h with coat of plates was 24/18.
Another thing to consider is how easy you would be able to trade your armour for another (in the future), so, if you really like that armour, I say you keep it.