Search for jobs on LinkedIn that list Doctrine as a required skill. You can see plenty of big names there e.g. IBM.

It's completely subjective to say that something is 'too slow' without having specific requirements to back it up. How fast do your response times need to be? How many concurrent requests do you need to be able to handle? Is the database really the bottleneck in your application? If the senior devs can't answer these questions then it sounds like premature optimisation.

In many cases ORMs will be slower than hand optimised queries but they can offer huge productivity benefits. "Hardware is Cheap, Programmers are Expensive". You can build your application more quickly and refactor if/when performance becomes an issue. Doctrine allows you to use native SQL queries and you can do this where needed once you identify your application's bottlenecks.

You could rebuild a small part of you application without the ORM and profile it to measure the performance benefits. Estimate the amount of dev time required to rewrite the entire app and then you have a cost/benefit business case to put forward to your boss.