It seems conceivable - either less drag and/or some elastic store/return of energy, but OTOH I'm not aware of marathon runners (where you might think it'd make more difference) wearing any special kit.

Could give a psychological boost though - a placebo effect - even if you only believe that it gives you an edge.

It is very easy to do the math on the aerodynamics, even at running speeds it isn't insignificant. It drives me crazy that many pro marathoners are in flappy clothes, or big hair cuts!

In a mile it could be between 1 and 7 tenths, depending on wind and and how bad the default outfit was.

And that may seem insignificant but its big margins at the elite level.