Re: A beginner with delusions of grandeur
When training with people of lower rank/lesser experience you will be expected to help them with clear instructions and demonstrations of correct technique. This forces you to take a closer look at what you are doing, think about the mechanics of a technique, and so increases YOUR understanding of aikido techniques. In other words, your sensei might not be asking you to do these things just because you are good but also because he or she wants you to take more time to think about what you are doing.
Not showing your true capabilities is not honest and is not in the spirit of aikido. Beware of false modesty!
I agree with James's comment that your competency is a reflection of your Sensei and other students' ablility to teach as well as your ability to learn.
I remember reading a saying somewhere, something like "On the Path there are always people in front of you, and always people behind you." Don't forget the ones in front! Also, I guess it would be true to say that people progress at different rates. These are not always constant, sometimes people seem to learn slowly, and then progress quickly. And vice versa. So remember that the people who are the same grade as you now, who you consider to be not as good, may one day 'overtake' your skill level. You may find later when introduced to more varations of a apecific technique that some people will be better than you at certain versions (eg: light, fast, flowing) and you might be better at others (eg: solid posture, breaking consolidated grips).