“These guys are experts at their field. Unfortunately not a perfect review because we ran into issues on the project - responsibility predominantly on my side, as I forgot/didn't consider to specify other items that were live on the server, which would be impacted.
I assumed they would dig around the server and find out what would be impacted etc, however that was a faulty assumption on my part, as they only work on what they start off knowing, and they can't spend a lot of time learning about a new environment.
Lesson learned - in the future make sure to draw project parameters *perfectly*. Understand what may be impacted, think of everything (write down a list, sleep on it, and review again to make sure you haven't missed anything), and then provide full specs/details to contractors, so they can actually do their work, and all bases are covered. It helps if you aren't rushing to get the job done before a full-blown site re-launch in a few days.
A case of poor communication on my part in that regard.
Unfortunately to compound matters, part of the optimization didn't work perfectly - they installed Varnish which proceeded to crash overnight, so there were likely unresolved issues, or there were bugs that needed to be ironed out. Unfortunately it was the last straw for the client, particularly because the optimization was being performed on a production server (yet another mistake on my part - always make these sorts of changes on a development server *first*, before rolling out to production).
If the contractors had been given more time, I have no doubt they would've ironed out all bugs, and the server would've been optimized 100%.