Does role priming ("you are a senior Magento dev") actually help?

Marginally. The bigger lever is task framing: tell the model what the output will be reviewed against, not who it is. Compare "You are a senior Magento dev. Add a plugin to product save." against "Add a plugin to ProductRepository::save. The PR will be reviewed by phpcs --standard=Magento2, phpstan --level=6, and Adobe's EQP checklist. The Marketplace tech-review reads it next." The second framing produces strict-typed code, full DocBlocks, no ObjectManager calls, and proper exception handling, without ever saying "senior". Save the role priming for the sub-agents (e.g. a code-reviewer agent primed as an EQP reviewer).