Driving in Sport mode holds the gears longer for quicker acceleration and whenever you slow down/stop, the transmission will always go to gear 1 (sometimes when you slow down to crawl, normally the transmission keeps it in 2 for smoothness). Sport mode will do nothing to your engine's wear-and-tear, feel free to use it as often as you like. Don't be afraid to take your engine to redline...engines just don't care anymore how fast they are revving. They can chill at like 5,000rpm or 6,000rpm (assuming a 6,600rpm rev limit) all day long and you'll still be fine.
I can personally attest to this as I whip the shit out of my car every single day and the engine absolutely loves being above 4,000rpm+.
BMW BELIEVES in naturally-aspirated high-revving engines...when you are driving this car hard, BMW WANTS you to keep the engine at 3K+ or 4K+ rpm's...that is where all the power is, and that is where you will have the most fun (but the worst fuel economy).