If a performance based bonus is paid to an employee by March 15 of the year in which it is earned, is the arrangement exempt from 409A; i.e., can the parties dispense with the otherwise-applicable requirement that the deferral election precede the end of the performance period by 6 months?

As I read Notice 2005-1, Q&A 4 ©, this is a short term deferral that does not come under 409A.

What "deferral election" are you talking about??? What is being deferred?????
Are you saying that the plan normally pays out on March 15th of the following year but offers employees the option to defer that payment to a later date? If so, then the deferral elections under such a plan ARE subject to 409A.

No deferral past march 15 is allowed under the scenario I was asking about.

There is a priority in how you apply the rules. The first question is whether you have a deferral of compensation. If the answer to that is "No" because the amount is paid as soon as there is a legal binding right to it, then you stop and don't look at any other rules. It's only if there is deferral beyond that point and 409A does apply that you then consider whether the general rule on timing of elections is met, and, if not, whether the performance compensation exception is met.