Great letter. I like that its short and concise.
When you can send a letter I think its a great idea. It can give you closure, help you feel that you have made an active and final break from this group. Further you may make an elder or two think. Maybe even a CO or two. It depends on how much discussion takes place. I know I got my first taste of "apostate" gripes as an elder when other elders referred (admittedly derogatively) to their complaints. It got me Googling the issues and here I am. So putting your concerns out there can help in more ways than one. But of course once you do, you are ostracized. There is no turning back. Keep that in mind.
Isaac