AZ, I could have written your post. I am *exactly* the same kind of rider, the only difference being, I am lazier! I have thought of getting a coach at times, but I don't ride with a group like yours. The main group I ride with is slower than me, but there's a growing segment of this group, who call themselves spirited. I rode with them twice last year, and while I did it, I felt like I was going to die for a day after the rides. They always encourage me to come with them, as I am clearly faster than the main group.
I say if you want to spend the $, try it. Me, I think I'll just make DH crack the whip on our rides. I cry most of the time I'm in those situations, but it works.