Change in transformer efficiency (leading power factor) with increasing load current
Does efficiency increase or decrease when load current increases
  (assuming copper losses equal iron losses, leading power factor load)?


The simple scenario of zero load current has to imply that output power is zero therefore power efficiency MUST be zero. So, taking a little bit of load current MUST cause efficiency to increase because you have an actual output power (assuming the load is resistive).

If the load is leading by 90 degrees then under no circumstances will power be consumed therefore efficiency has to always equal zero. 

However, if the load is somewhere between resistive and capacitive (i.e. complex) then any current taken will produce an output power and therefore efficiency can be seen to increase.

At some point there will always be turn-down of power efficiency but without a real scenario with a fully defined transformer equivalent circuit and load it's hard to say where this turn-around will occur.