I've disabled DHT because the traffic (noise) it generated outweighed the importance of it (especially when the trackers are up), but I still see in the log that I get new peers via PEX, and I wondered if this is possible to turn off, or at least if it generates any substantial amount of traffic.

It can be turned of per torrent (right click on it -> additional peer sources). It will generate a slight increase in traffic, but not that much.

Do you think we could have the ability to turn it off "globally" like DHT?

OK, why not, shouldn't be much work