Is the efficiency of (Qi) wireless charging affected by a metal surface?
Your phone and the charging device form a 'magnetic circuit' across which power is transferred.  It is possible to 'short out' the magnetic circuit, particularly with steel.  In the comments, alex.forencich suggests the easiest solution:


  put the phone and charger on top of a stack of magazines, books, etc. and compare the performance


I can guarantee that the same steel plate between the phone and charger will create a magnetic short circuit and very little energy will get through to the phone, but it is difficult to determine if a piece of steel under the charger would have the same effect without knowledge of distances, thicknesses, materials, etc. that would be difficult to determine without some teardowns.

Let us know how the experiment goes!