@PedroRodrigues currently there are no SLA management tools for tickets. Using workflows as you have done is the closest you will get to this.
We are considering building an SLA tool whereby you can setup an SLA and then link it to a company or contact record. And then have a CRM extension to notify the agent/team of the SLA relevant to the customer is question, as well as notify them when they SLA will be breached.
We are assessing the demand for a SLA tool like this - would you be prepared to contribute to the development costs of such a tool?