Those sizes sound like alternator pulleys to me. If so, I wouldn't mess with it.
Changing a crank pulley...
Yes there is a benefit. You will gain power with an underdrive pulley. Since you have an M6 you can easily get away with 25% reduction underdrive pulley. You will have to get a different belt. As far as the aluminum, I don't know.
Changing the pulley is really pretty easy. Check out installuniversity.com and go the the cam swap how to and it shows exactly how to take the pulley off. It might be a good idea to change the front main seal also which will require you pulling the water pump and front timing cover. Changing the front main seal is only added insurance.
If you are only changing the pulley just get a good 3 arm puller to take the stock pulley off and DEFINATELY BUY A LONGER CRANK PULLEY BOLT TO GET THE NEW PULLEY ON. M16 x 2.0 pitch x 120 mm long and also two washers.