Paul,
I recently sent a document to the Visio team (and some of the other MVPs) that looked at some of that. In my case I was noticing a heavy degradation in performance occasionally, and it turned out to be the result of 'some' add-ins. The key was whether the add-in had been created with vsto. The thing your developer contact has to be aware of is even though he is using the activex control, visio is running under the covers in the last configuration it was used in. In my case to meet the performance I needed I had to ensure that nobody's add-in was running in the Visio environment. In any case it sounds like your add-in and his code are arguing about who handles which interrupts.
al
