I drove very much Timetrials in GT5P. Over 160 i think. I hold 44 Worldrecords or something. at the moment Some Records i hold with clutch, some i hold without clutch, and some i hold with clutch mod (dsg mod).
YES, definetely its an advantage to drive with clutch, BUT at first u need the skills to handle the clutch.
It´s not as easy as it seems. Clutch driving also got disadvantages:
1st is, u cant break with the left foot, because u need your left foot for clutch pressing.
2nd sometimes u will miss gears if u change the gears too fast.
so what happend if you are on a good lap and you miss one gear? u are ****ed

.
the best thing on paddle driving is, u have always 2 hands on your wheel ... and u have autothrottle while downshifting. with clutch u need to do heel/toe technique - its hard to perform, but its possible. i have done all of my clutch times with heel/toe.
the time advantage per lap is hard to say overall. it depends always on the combo - car and track.
example: with the f430 on suzuka u have very much shifting processes per lap. my best lap with paddles was a 02:10:190 or something... and my current best with clutch is a 02:09:960. the difference is about 2-3 tenths per lap.
on london is the clutch more or less a bit of a handicap. the course is very tight and short... u have not really much shifting processes there, so not a big time advantage.