i have 20 feet of tsunami comp series oxygen free 8awg wire laying around and i am thinking upgrading my ground wires on my 89 XJ with it?

this wire was left over from one of my amps and i want to know if 8AWG wire is good enuff to use for a ground

I don't know how much power your amp uses but for the rest of the Jeep, upgrading your grounds starts at 4 AWG (stock cables off battery appear to be around 6)

For a ground you can't really go too big I got a 4 gauge battery cable and just mounted it to the extra bolt by the alternator bracket and the to the frame behind the right headlight. You will obviously have to sand off some paint but it works well.

No, I wouldn't consider an 8AWG wire an upgrade.

Need to go with 6AWG or more, I also think the battery cable's are 6.