Splitting ACKs such that one ACK acknowledges less than a whole packet has been reported to cause senders to increase their rate faster than it should (in accordance with the rules of congestion control). Appropriate Byte Counting (ABC) is a common remedy, but not all senders correctly / fully implement it, in particular in Slow Start. We have found that it is therefore possible to provoke some senders to send out a larger number of packets than they should within one RTT in Slow Start. This could be useful for measurement purposes.
This work was developed in Rolf Erik Normann's master thesis, which is available here:
A shorter description is given in:
Michael Welzl, Rolf Normann: "A Client-side Split-ACK Tool for TCP Slow Start Investigation", accepted for publication, IEEE ICNC 2012, 30 January - 2 February 2012, Maui, Hawaii.
and the tool is available here.