It was clearly mentioned that the difference on ECC and BCS lies in different types of the systems: OLTP & OLAP. And first of all, this not only from the reporting point of view. If you want to make a step back – to ECC, then get ready to redesign ALL the business processes that may affect consolidation. Namely, ALL the processes in the core modules: FI, CO, MM, SD (the list is not complete). Do you need this head ache? It will not be even a new implementation project. But much – much more...
In BCS you may load into the consolidation system just one (for example) row of aggregated data instead of probable VERY MANY transactional ones.
And the reporting part of consolidated reports (OLAP – vs OLTP) may also play VERY significant role.