Can you email me at adam at heroku dot com and I'll connect you with one of our Python experts? I can't promise we'll solve it, but I'd like to take a look.
Would you be willing to pair up with one of our developers on your app's performance? If so email me (adam at heroku dot com).
Fwiw, I've found the addon / db connection limits to be the primary blocker when load testing so far.