I don't have the roti, but I have cooked my fair share of chickens - whole vs spatchcocked, direct vs indirect, main grate vs upper grate, vertically (NOT on a beer can!!!) vs horizontally, and I have to say that there isn't a lot of difference in the juiciness of the meat between all these variations - all come out succulent, but the differences are in the crispiness of the skin and the overall cooking times. Obviously, a spatchcocked chicken is going to cook faster than a whole bird, either vertical or horizontal on the grill, as there is more surface area directly exposed to heat. I seem to get better results on the skin doing them spatchcocked, too, as it is easier to control the browning of the skin by being able to flip it over either toward or away from the heat source, as needed. It also has the advantage of allowing the use of the upper grate to get the bird closer to the dome walls without worrying about burning the high spots on a whole bird.
Here's some pics of what I'm referring to.
First, the whole bird on the vertical rack (can't do this on the upper grate).
Got a little too done on the very top where it gets close to the dome walls.
Now, here's a spatchcocked one on the upper grate.
By comparison, a horizontal whole bird on the upper grate.
While this one has great color, the skin isn't as crispy as the spatchcocked bird.
Unless I'm doing Cornell or Alabama White chicken, I like doing chickens spatchcocked, direct, on the upper grate @ 425 dome, the best so far. With those sauced/brined chickens, you really need a drip pan to stop the flareups. YMMV.