A big consideration I would have is that the downdraft type exhausts
aren't very effective. My house came with a Jenn Air range that has
one in the middle. It works and does catch some of the smoke, it's
certainly better than having nothing or a recirculating one, but it's
maybe 50-70% of what a real overhead hood will do. The problem is that
is you have a pan that is smoking, the fan just can't suck all the air
sideways, so some of it goes straight up. If you use the range in
ways that produce considerable smoke, like blackened fish, you're going
to still have enough smoke to set off smoke detectors.
So, I'd consider how much longer I'll have that settup, when you might
upgrade the kitchen, and what the other alternatives could be before I
did a lot of work and made holes in walls, etc.