Long story short for the past 3 weeks I've had to go from a 2.6 pulley for which I had the car dyno tuned for to a 2.8 pulley because my 2.6 mysterously broken and took out my belt. My question is how much damage if any am I doing to my car by running the bigger pulley?

 

It will run a bit richer.

 

^ Basically yes, which could be a bad thing if its TOO rich. It all depends on your tune. I would recommend you either get re-tuned or get another 2.6 if your first one was in fact just a freak accident.