Hi Andy,
Thanks for the speedy reply.
There are a few reasons that we haven’t made a move to the Account Activity API. Firstly, it would be a lot of work to move from our current polling architecture to a streaming architecture as our system is quite large and has many customers.
Perhaps more importantly, we have 9000 active users and I think using the Account Activity API for this many users would be prohibitively expensive 
Is there any way we can apply for an increase to the 15 calls / 15 mins rate limit for the DM events endpoint for our application?
Thanks,
Brett