Better parallelization than a sequence container in SSIS 2008?
Assuming I understand the situation, you have lots of source data to process but it all needs to go through a slow destination (stored procedure) then there's a native component, Balanced Data Distributor, that might be of interest. 

As a row flows into a BDD, it can detect whether the downstream consumers of data are busy (whether there is backpressure). It's somewhat similar to a Multicast in that you can have multiple output paths but you won't have duplicate data flowing into them. Instead, it'll be using backpressure detection to know whether a worker is available. That should address the issue you're encountering with your manual segmentation of the problem.

It doesn't come preinstalled with SSIS so you do have to work through that but it is a MS supported component.