How signficant is leakage inductance in a flyback converter?
In a flyback transformer design you are storing energy in the primary then releasing it to the secondary. If you have leakage inductance, there is energy stored in the primary that doesn't get coupled to the secondary. This uncoupled energy has to be "burned off". This is where inefficiencies can be seen.

If you have a coupling factor (k) of 0.97 and a primary inductance of 38 uH, the leakage inductance is 38 uH x (1 - \$k^2\$) = 2.25 uH.

Given that primary energy storage is all to do with the peak current (squared) in the charge cycle, and this current is common to coupling and leakage inductance, then the "loss" of this circuit (ignoring all other losses) has to be 2.25/(38-2.25) = 6.3%.


  How signficant is leakage inductance compared to a series DC resitance
  of 0.07?


There is no information in your question that allows this to be computed.