If I add new pistons, cams, valves, and valve springs do I have to have a tune done? What would be the positives of it?

 

if there OEM parts then a tune is not really needed if your upgrading to bigger camshafts, pistons, etc. then a tune is highly recommended

 

I've used all OEM parts and still needed a tune. A tune is meant to maximize the engines power potential with a proper air:fuel ratio as well as proper timing values.

Even an internally stock GSR with bolt ons will benefit from a tune.

 

You might, I rebuilt my Type R motor which ran good on my stock type R ECU BUT when it was rebuilt using the same parts the motor ran very rich so it was tuned right after, good thing i was at my tuners shop.

Invest in a AFR gauge!! this is VERY important and a great gauge to have in your car. Tells you whats going on with the fuel trims. So hook it up and start the motor on the stock ecu and let it warm ect and monitor your AFR you want to see any where from 13.5 - 14.5 at idle and part throttle and 12.5 at full throttle, if your running rich like 10-9 at idle shut it off and have it tuned asap. You don't want it to run extremely rich during start up and idle to avoid washing out the rings but this usually happens if it is very rich and runs like that for a while.

 

Yes, if you add new pistons, cams, etc. it will need tuned.
The positives? Your engine will run right, make the power its supposed to, and get the best possible MPG. As long as the tuner knows wtf they're doing...