I honestly wouldn't cover the foam with a piece of plastic. Putting a layer of plastic (whether sintra or wonderflex) over the top will look obvious, and may be too thick - it won't look right. If you're thinking the plastic is a quicker or easier way to get the smooth finish before painting, I would say it's...not, really. It's more expensive, for one, than simply coating the EVA foam with something that can be painted. For two, trying to heat-shape the plastic to fit the exact curves and shapes of the EVA foam underneath is going to be an exercise in frustration. I say this as someone who regularly heat-shapes sintra over the top of other materials. For three, wonderflex doesn't heat and shape very evenly, you may end up with fingertip dents all over the plate.
If you use the plastics alone, you're going to end up with much thinner plates of armor. It may look more like the character. If you use foam at all it should be just to bulk up the legs, if your legs aren't that thick. The armor plates are fairly thin (standard-sized, anyway), they just stick out from some kind of under-layer support that makes them stand out from the legs a lot farther than just a pair of pants. Your under-support won't necessarily be painted or even seen for that matter, it can just serve as a place for the armor plates to be attached.
verdatum, that joke was so timely you are entirely forgiven.