I don't agree with this at all. A huge reason a lot of 10-13 (or so) HDCP's are missing greens is because they're in the fairway off the tee but hitting into longer par 4's or long par 3's with a long iron instead of a shorter one.
Adding 15 yards off the tee will equate to added GIR. Also, in theory, the same player could be taking one club less with increased speed. In essence, adding speed and gaining distance off the tee, taking less club as result AND being able to hit irons longer is probably equivalent to taking two clubs off. More GIR, lower HDCP.
I agree with the premise, though. I just disagree with the measurement. Its not HDCP or GIR..its quality of strike. If you can swing the club efficiently on the right path and make solid contact, you can benefit greatly form speed training.
Its probably where I am at the moment. On a shorter course 9 hole course I can score in the 30's on the regular. Mid-single digit. Thats where I am now playing the course I routinely play. Get me on a course with some yardage and I'm gonna struggle and that 6-7 is going to shoot up to a 12 pretty quickly. But if i could add some speed/distance, I'm capable of having that same mid-single digit on that course as well.