Depends. Personally, i was at max plunder with just about an 8bil ally. So just find out by hiring cheap allies and hitting the SAME eb or person and making note of the ally plunder
Volley with the other person until you run out of gold, and the person transferring keeps the ally giving youa boost.
I've been converting my t3 builds to t4 builds... Should I stop now and upgrade my 10 t4 builds, or keep converting?
A rule of thumb should always be upgrade to lvl 3-4 before moving to the next one. It gives more then two lvl 2 of the same buildings.