Stick with the AC Delco factory plugs and wires....AND GET THE WIRES!!!! Good chance your going to break them coming off anyway. Advance carries the factory replacement with lifetime warrantee. Just changed mine out and gained 2 MPG running empty, towing run this weekend.
Computer tuning.....1st what engine do you have? if its a 5.3 L33 forget any plug in box tuner, custom tune is the only way to go. Have then do away with the torque management system. Add a cooler thermostat and have them calibrate the fan temps.
As others said, synthetic oil change, clean air/fuel filter, clean your MAF, especially if you have an oiled K&N air filter. Changing the remaining drive train fluids to Syn may give a little more edge also.
2015 Shamrock 21DK
2005 Silverado Ext cab
Nights camped 2016..... not enough