Overdrives are great especially here in the States. Loooong roads mean higher revs, lower fuel economy, and even quieter cabin wo Overdrive.
As others have mentioned on twisties, going in/out of OD is wonderful, you don’t have to touch the clutch.
I think a Mk2 GT6 will have a D-type overdrive. I believe the D-Types build and maintain hydraulic pressure so you have instant Overdrive. You may not notice it’s in OD because of it. The later J-Types build pressure when engaging with the switch so it takes a sec or two. Almost annoying when left on all the time.
Have fun with your new toy!
Todd