but consider the exit of the Iris, you have an opening of say 1 inch in a 3 inch pipe, yes it's in the centre but its like falling off a cliff when you exit it

the air passes through a 1 inch hole into an immediate area of 3in, the turbulence at an exit of an iris would be about as bad as you could get

i think you have grasped onto a small portion of good air flow principle and are not considering the factors which come in to play

yes the air flow is faster in the centre of a pipe and yes there is friction on the side walls, but you need to consider how much air your flowing and how you are regulating it, the side wall friction is only going to come in to play if you are flowing a high amount in a small pipe... the thing is this is taken into account by throttle position, if you are on part throttle and your not getting enough air flow due to losses in your throttle body the simple solution is to open throttle body a little bit more my pressing your foot down a little more on the accelerator..

(btw - i agree butterfly is not linear, just saying it is more linear)