Maybe if you were talking about building a sniper rifle where you are settling in behind the rifle from prone, or a bench... a lot like snipers, highpower shooters, and scoped M14 shooters do. But I because... at least for the moment, we are talking about putting an ACOG on a SIR, and SIRs only fit on carbines, I presume we are talking about a running and gunning CQB weapon. In which case the gun will see a lot of fluid movement and fast paced shooting. I dont know how much of this you do, but I can tell you one thing... things like the correct shooting stance and position are the last thing on your mind. In this scenario, Im unsure how a conventional cheek weld is going to help to any descernable degree in any way shape or form.
On the other hand, I can give you a list of reasons how an ACOG on a SIR will help you on a CQB gun. I just dont see why a half inch of movement in your head should keep someone from using one of the best handgaurd systems available, with one of the best optics available, using one of the best mounts available. I cant say it will work for everyone, but saying lifting your head up a little will degrade your accuracy is quite subjective in my view.
Put it this way, there are probably many more things on most rifles that will inhibit a shooter from acquring the most accuracy out of their gun than a *slightly* elevated cheek weld. Like a stock trigger for example.