Keep in mind that longer distance increases likelyhood of partial cluster. It is far more easy to lose half of your nodes if they are in another building than if they are in another rack. In this case if you lose quorum, your cluster will be in severely restricted mode, and if outage is prolonged you will have a problem.
That is the reason why stretch metrocluster exists. You have to weigh in all pros and cons and understand impact of cluster split.
This is not decision that can be taken based on simple math and cable data sheets.