There are plenty of high mileage LS4 cars with DOD, it's not like there is something magic that will fail at exactly 60k and make the car un-drivable.

I do agree with doing a tune based on timeline of other mods. You want a tune as dialed in for your setup as you can, so if you're going to do more big mods soon- it may be a better idea to wait.

Depending on who does the tune, gaining peak HP isn't usually the biggest reason- especially at your mod level on this platform. Some settings can be modified (mostly in the transmission settings) to make the car feel more alive. The factory has to live with constraints for NVH, fuel economy, etc- but a tuner can improve the 'feel' of the car every day for someone who is a car enthusiast. That part (IMO) is more rewarding than the small gains experienced at WOT.