From my benchmark, I can tell even in exported games it makes a difference, probably because GDScript lacks of a few straightforward optimizations at the moment.
However, the impact is very small. I have a test doing
a += member, where
member is either expressed as is, or with
self. Executed 1000000 times, the result is that the version without
self takes 45 seconds, while the version with
self takes 47 seconds (I took care of removing the cost of iteration itself).