Looks like I'll be using a hybrid approach of the HTTP client, and a custom class running on JAVA to do the aggregation emulation.
Here's a recap of the consequences.
The first points of failure are (since the HTTP client for Sesame works great):
* SPARQL/Sesame not having Aggregate functions
* Sesame not having ORDER BY
This produces large amounts of results and/or queries, which then need to be parsed by JSON, leading to the second point of failure
* Zend JSON and native php 5.2 JSON are not fast enough (perhaps they should not be expected to be for 6000 results)