Hey bob!
I got my bondtech upgrade 2 weeks ago, and had it installed and testing within an hour. I used tinkergnomes firmware and agree with @eldrick, seriously good updates.
Within a day, I was printing at ~80mm/s, where before I was printing 30-40 iRobert setup.
As you mentioned, the steel coupler would be a concern, and in fact I had an issues with the feeder being TOO strong
I had a scary event trying to print too fast, and had to clean it up. (luckily didn't ruin my olsson block). This can be avoided by slowly adjusting the feeding settings along with close watching the print and flow.
I too was waiting for the printer upgrade from ultimaker, but decided I couldn't wait any longer and figured the bondtech feeder would be stronger. (I didn't have tested proof, just a guess).
A few reasons I like it though:
1. Its far less money
2. It lives outside the box, meaning it can get external airflow and hopefully not overheat
3. I could possibly move the position of the feed above the printbed for a more direct feed in the future.
4. Im trying full ninja flex later this week, should have some results to share, but previously was only able to do semiflex!
Hope this helps