Sorry for the late reply, but my alert feature is not working. I agree 100 percent and will add, you also need a free flowing exhaust to get it all. And I think it looks cool.Adding that extra stack was a smart thing to do as I'm sure it helped even out the grill temps.
My main reason for sealing the lid was simple & intended to help keep external staining to a minimum.
I can see that being a breathing problem with a large bodied single stack grill (especially at high temps) but probably not the 590 which has 3 exhaust ports as I suspect it keeps more accurate overall grill surface temps.
All we can hope for is a happy medium when it comes down to intake fan & air pressure rates thru the firebox & into the grill surface & smoke/heat exhaust stacks while trying to maintain fairly even temps for those long smokes.
I build high performance motorcycle engines & know from years of shop time with a flow bench that you can add all the camshaft lift, increase fuel etc. but correctly ported heads are the real icing on the cake that makes it all come together & perform on a dyno.
My point is, I'm sure somewhere along the way the engineers who designed RecTeq, Traeger, & all of the others have played around with intake & exhaust venting because anything can be improved on.