I considered that, but at the same time, I dislike the idea of dumping all of that hot air straight into the case. I thought with 360 as exhaust and 3 fans on intake (counting 240mm rad) thermals might be better on RAM and VRMs (including those on the GPU. That being said, having a 240 rad and 140 fan exhaust with 360 rad intake might not be too bad.
It just seems like a pain in the ass to test... I've considered selling my spare 240 rad and going for a 280 because for some reason I have a crap ton of good 140s laying around and more surface area is always better
Cold air intake through the front 360 rad, the exhaust through the top 240 rad. The air into the case will not be "hot". My temps are really good using this. Plus the front of the define S has a dust filter. So using any other intake will also add more dust into your case.
I'm not too worried about dust but you are right. If I do decide to whack a 280 in the top, I may just leave the rear bare to keep positive pressure. I haven't even bought anything yet, so I'll have to wait and see