03 4.8 65k Miles Should I replace wires and plugs

As the title says this is what I'm wondering. The truck runs great. I get about 13MPG commuting, the same as when I tow. I used to get 16 around town. So I'm thinking it's time to swap out some parts that could probably use it.

Any online spots for AC Delco parts with good prices?
I also run a K&N air filter which I clean regularly.

Any quick and easy ways to add some more HP?

I get ac delco parts from rockauto.com.... I just replaced the plugs and wires in my 04 HD Sierra. Made a big difference!! But 136 thousand kms on original plugs and wires isn't to bad LOL.