I can remember doing overburn tests using a 2x writer, and it WAS NOT 3-4 minutes .... you've never had it so good!
I recall one report, from a newsgroup, of an audio CD overburned to virtually NO leadout (6 seconds) that damaged a PLAYER - I supect it may have lost control when the leadout ran off.
All CD-R media has at least 90 seconds of leadout space, probably with some surplus, but the quality of this is uncertain.
You can often scrape up to 30 seconds margin without even running out of leadout space - if you have 90 seconds left out of the test reported space, you will write a full leadout, you will not error, and the process is probably safe for both writer and reader.
Once you impinge on the 90 seconds leadout, you WILL error during the leadout writing, and the risk increases - so how little is enough?
I think Diskjuggler has an option to reduce leadout to 30 seconds - and I'd suggest that's probably as low as you should ever go!