It may have some effect but if, in practice, all you can do is unscrew it I don't think you'll get the improvement you're looking for.
This is actually from my Webley Typhoon - that is essentially the same gun isn't it - and assembled outside the trigger mechanism housing you can see the screw in question and how it bears on the action:
http://i301.photobucket.com/albums/nn75/IainKD/02-1.jpg
Possibly the critical area to lubricate (or maybe even polish slightly?) is this point:
http://i301.photobucket.com/albums/nn75/IainKD/01-1.jpg
General dismantling is achieved by the two screws either side at the front, and one at the very rear underneath.
Hope this helps,
Iain