Selecting 'S' increases the revs by about 1000 for the same road speed putting the power output up nearer to peak output hence increased performance.
'S' is very useful for joining motorways, roundabouts, entering roads at junctions an of course overtaking.
You can switch between 'S' and 'D' while on the move.
Running in 'S' mode will increase the fuel consumption but not drastically, you can't have increased performance for nothing.
I use 'S' mode regularly, it's there to be used and in my opinion better than playing with the gearshift paddles if you have them'
Vic.