Development of ownCloud sync performance optimization is currently oriented on bundling multiple small files create/update/mkdir/move/delete into bundled requests. This makes sense for synchronisation of many small files in one sync or in the sync over the bigger latency. This work is currently in progress on both client and server side. You can read about that here:
The above improvement is called Bundling. If you are interested, I could append you a presentation I did about that for ownCloud conference. It is already implemented and under the tests now. This way you could have a sense on how to make a improvement to the sync algorithm.
About the improvement you are talking about, it is called Delta Sync. However, it is not easy to find a use case for delta sync, since e.g. documents are xml basicaly, it means each time you change them, their content is changing. It is nearly impossible to find a case for a file that deltasyncing will make sense (and it is very expensive)
What we are currently looking at, is to implement improvement called Dynamic Chunking (Dont mess up with deduplication using dynamic chunking). Basicaly, concept is that, in current implementation big files e.g. 100MB are being chunked into smaller pieces called 10MB chunks. The problem here is that this is fixed value, and it is not appropriate on all types of the networks (WiFi versus FiberToTheHome versus LAN). For WiFi it makes sense for small chunks, while for fast networks, it makes sense to have very big chunks.
I am not sure how familiar are you with TCP, but there the concept it is the same. It is called Additive increase/multiplicative decrease (AIMD) congestion control. This behaviour is also called Probing For Bandwidth,
TCP congestion control - Wikipedia
I would look for something similar, but for synchronisation of files. 