This morning, I loosened up the throttle housing and pushed the grip in as far as it would go. I saw that it was going to stick - so I moved the grip back out a bit and tightened it back up. Next, I found the sweet spot on the throttle cables. I rode today and the throttle play is gone. Much better. I didn't need the spacer.

I've had the isos and the throttle boss on for about a year. It took me a little while to get used to the feel and the looks of the throttle boss - but now I really like it.