Here's my understanding of the speed-up pulley. Somewhere in the production of the 44A and 50A mowing decks, IH started using a smaller center pulley. From 5ish inches to 4-1/2ish inch pulley. How I found about this was doing a goodly bit of searching on IH decks and their quality of cut. There's a bunch of info in the archives.
I came to my speed-up pulley by accident. With the purchase of my 1650 came a 44C deck (used on 82 series tractors and later) and the pulley on it was for a wider belt. I blindly ordered a new pulley and what I got was the speed-up pulley.
Fast forward, I bought the 109 and got it mowing with the 44A deck that came with it. The idler pulley ate the deck belt and when I changed it, I also installed the speed-up pulley I had on the shelf - for the 1650's deck.
I think it makes a difference. Prior to installing it, the clippings exited the deck kind of just dumping out the chute. Not bad, but nothing like my other color tractors and especially not with the velocity of my MTD CC LTX4042 - which is a cheap tractor with CC colors, but gives a GREAT cut. That machine throws soaking wet clipping out with ease and no clumping.
Back to the IH content...
So I installed the speed-up pulley and have mowed with it once and the velocity of the clippings out the deck are definately faster, but again not as fast as my other tractors.
I need to do some side to side adjustment of my deck and I think I'll have a pretty good cutting machine with the 109 and the 44 inch deck.
BTW, I also installed new blades. From the archives, at least one member is using Gater blades with very good results and the speed-up pulley.
I use my tractors 99% of the time for mowing and I was/am anxious to get these IH tractors giving me the same quality of cut as my other color tractor tractors, hence the tinkering with pulleys, etc. My view is IH built a premium machine and they should cut with the best.