Theoretically yes, and if you're the unicorn, then I'm sure you are very appreciated (and probably under-compensated). Seriously. I'm not mocking you.
With a person who's job is to help you and who is not above you on the food chain, you are more likely to have an honest self assessment. Also, with a third party in place, fairness becomes part of the equation. When a developer has to defend his timeline it's personal and can be very stressful. When a third party has to defend the same timeline it's just a matter of business. Conversely, when a third party is in play that is told that a developer needs to produce more, their instinct is to prioritize accordingly rather than simply add more hours to the work day (which is a common instinct among developers).
With a person who's job is to help you and who is not above you on the food chain, you are more likely to have an honest self assessment. Also, with a third party in place, fairness becomes part of the equation. When a developer has to defend his timeline it's personal and can be very stressful. When a third party has to defend the same timeline it's just a matter of business. Conversely, when a third party is in play that is told that a developer needs to produce more, their instinct is to prioritize accordingly rather than simply add more hours to the work day (which is a common instinct among developers).