I was at a Ryan Home last week that had one of the double wall ovens. We were cooking dinner together and waiting for the bottom oven to preheat to 400 degrees. We waited over an hour and still never got there. I was told this was "the norm" for their bottom oven. They were told it was going to be slower and there wasn't much that could be done. How frustrating.
I don't know if you upgrade the wall oven from the standard if it works any better or not. But if you are considering getting the double wall oven, I would talk to your rep and PM. Find out if upgrading will help the performance of the bottom oven.
If you read this blog and have a double wall oven, I'd love to hear from you. How does your lower oven work?