Last years turkey notes, and questions about this years planned adjustments

Sea2Ski

12.1 Lbs Spatchcocked, with 2 additional legs and 2 additional drumsticks.
Lit BGE (Lg) at 9:00 AM stabilized at 350 dome. Put bird on at 12:15, direct, at felt line with one handfull of pecan chips.
Additional parts placed on second grate level.
Took off at 2:42 PM. Last 20 mins temp raised to 400 for color. Breasts were 154, bird legs 177 Leg parts done 10 mins earlier @ 185.

This year, we are having a few more people, but repeat attendees are mandating a repeat performance.
A repeat is fine, but I want to do better. So I am asking for your thoughts on my planned adjustments.

This year, I have a 17.84 Lb bird. I am going to spatch it, but I believe I will need to drape it over an inverted V-rack for it to fit.
@SGH‌ mentioned injecting butter into the breasts. This sounds intriguing. Since I dust with Dizzy Dust Shaking the tree, I was thinking of adding a few shakes into the butter. Question: Should it be unsalted butter? Or regular butter?
Also, since cooking time is not linear, I was wondering how much longer I should expect a 5.5 lb larger bird to take. I am thinking about 1 hour longer. Does this sound correct? I know to cook to temp, but I am aiming for everyone to sit down 1:30-1:45'ish. Thoughts?

theyolksonyou

Standing by for some answers here as I have similar questions. Thanks for posting.

Mickey

I always do 12/13 lb birds at 400 raised direct and takes app 75/90 mins. This year 18 lb bird and thinking 2.25hr + or - . I use one handfull chips, half pecan & half cherry.

Mickey