In current implementation Traffic Server uses a two level hash for keep-alive connections: server ip -> hostname -> keep-alive session.
The reasoning behind this was that some old http servers could not handle the request properly for different virtual hosts on the same keep-alive session. Recent web servers should all handle this situation correctly.
The improvement would be to make a single level hash to keep just the server ip. Shared hosting configurations using ATS as a reverse-proxy could benefit the most from this improvement.