Insular Electronics, a company with 15,000 employees in six metropolitan areas, (has/have) been a leader in the local economy for three decades.

Different English speakers treat this differently. The fictional company, Insular Electronics, is a single entity, but is made of multiple individuals. In the US, we stick with the entity and use the singular verb, so only "has" is acceptable. In the UK, "have" may be accepted as well.