Reg Edwards wrote:
A choke balun is what is inserted between the transmitter end of the
antenna feedline and the transmitte tuner. Or it can be inserted at
the antenna end of the line.

Questions : How effective is it? To what extent can the percentage
unbalance current on the feedline be expected to be reduced? Has
anybody ever measured it before and after insertion?

Yep, and you'll find it right on the web. Go to
http://eznec.com/Amateur/Articles/Baluns.pdf and read the section titled
"Experiments" beginning on p. 3 of the PDF document. You might also find
some other interesting information in this article.

Warning, though. It was written by an American, so should be regarded
with great suspicion and skepticism.

Roy Lewallen, W7EL