We have the "Exhaust" smell after medium/hard acceleration as well. 2014 Ex LTD 4k miles so far. Had the same problem with our last truck, 2004 Sequoia. Problem was never resolved, they just replaced the "cats" with some that didn't create quite as bad an odor. I experimented with a longer exhaust tip(single exhaust,bigger(4.7v8) engine and it made a significant difference, but did not totally eliminate the problem. I figured when we bought this truck that SURELY all the manufacturers had resolved this type of problem. Ha!. The design of the exhaust on the Ex is at least part of the problem. You have a 213 cu.in. engine which really doesn't need dual exhaust(it's not dual all the way anyhow), then you split the volume of the gas into two separate pipes at the very end of the run, then you flair the pipe to a larger size, THEN dump the now slow moving gas into a larger "tip", and there is very little velocity remaining. Add to that the vortices around the immediate rear of the truck, and the gas is going to get sucked in anywhere there is an "opening". I have noticed that it does not happen at lower speed short bursts, but more so with longer periods of acceleration, like from 55 to 75. I have also noticed it even when going over an overpass on the interstate, which causes a downshift while on cruise control.
Running the A/C on "fresh air intake" helps by partially "pressurizing" the cabin with air from the front intake. Just my opinion, which doesn't get much attention. We were told last week, that they "just got a TSB" on the problem, and they were going to do the fix on a 2014 Ex that belonged to their dispatcher. Prior to this, NO ONE at the dealership had EVER heard of "the problem" before.??? I can't find any mention of a "new" TSB anywhere, but, we will find out more this week. Is the problem in ALL explorers? or is it that some owners ignore it, or never accelerate at a rate that produces the problem? I'm hopeful, but not excited,,yet. We really like the truck(not really a truck anymore), but are very disappointed that this problem has not been resolved when there is evidence that it has shown up in all the "5th gen" models.