I don't see much about these pipes when I search. Do they offer any increase in power and/or sound?

Description from h-d web-site:
Harley-Davidson®Softail®Slip-On Mufflers, Shorty Dual - Slash Down

These 49-state legal mufflers allow you to have an exhaust system with classic Slash Down ends while maintaining a pleasant, street legal sound level. Features Harley-Davidson®script on rear muffler. (Not legal for sale or use on California catalyst-equipped vehicles.)

Part number: 80378-00