This somewhat depends on the maximum download speed your connection will allow, and what you are getting at the moment .. but I would have thought you should get a noticeable improvement on a large file tens of minutes rather than hours
... of a few hundred MB... On a 5 MB file I think you will see no noticeable difference.
Your search results will be better .. as you will be connected to more ultrapeers .. if you connect as a leaf,... 5 connections instead of 3, and you will get a larger number hosts when downloading from multiple hosts, 10 instead of 8 (I think ), which will improve d/l speed unless you are already running at your connections maximum
I hope this helps