It's good to set expectation up front with each other, and a performance plan is one way to go about doing it. The downside to completely getting rid of a co-founder is:

1) They're likely going to take /some/ amount of equity with them (have a vesting plan!)

2) Getting rid of a co-founder before you reach critical mass will be detrimental to the business. If not, they probably weren't a good pick for co-founder.