I originally used 3" elbows (the kind that you rotate to change the angle) on my smokestack mod. I ended up rotating them so it just went straight down. I removed the warming rack.
In my opinion, it's not as good. Why? Because when it goes straight down, it's not at the front or back of the grill. It comes down on the left side of the grill, but in the middle (as far as front to back is concerned).
When it's in the middle, it basically means you can't use the whole left side of the grill. When the smokestack comes down in the corner, you can use the whole side except for that particular corner.
This was an issue for me when I couldn't properly fit 2 foil pans on there when I was doing the 3-2-1 method on some ribs. I now have the flexible stuff ran to the front corner, and I even have my warming rack installed.