Have someone perform a blower door test to de-pressurize the room with the complaining tenant. Then smoke test the interior and you will find where the air ingress is occurring. Then you can seal up the room, rather than try to alter the design of the HVAC system.
Note, if you go with the...
Why can't you just use the smaller 5 ton systems throughout the building with 2-position, or modulating, outside air dampers, along with a barometric relief (exhaust) on the return side of each system? Obviously the building layout would dictate if this is possible, but just a thought if you...
Can you post a picture of the piping schematic for the coil? But I do agree with posts above, it looks like you have air in the coil. The air must be bled out or the coil will not transfer heat correctly/evenly.
