Well, I've had a few more grape vines mature, and I can say without a doubt that the growthcraft grape vines are by far the easiest method to produce food at the iron stage. It's not as OP as the fishtrap was, since you can only get grapes from the vines (as well as make "potions" later on with a fruit press and fermenting barrel), but I have produced quite a few grape salads. At this point, I can produce more grape salads than I consume puttering around and building out my base. My 6 grape vines still aren't fully mature (although rather close).
As far as OPness goes, I think the grapes are produced at a speed on par with the natura berry bushes, but you can't plant the growthcraft grapes without finding vines, slimeballs, and creating a TiCo smeltery as the ground needs to be tilled. So, they come a little later in the game, but honestly, by the time I hit copper and iron, I'm not really too worried about food. I'm still living off foraging normally at this point as my HC farms aren't fully operational, but since I have all the HC cooking utensils, the food I do forage goes a lot further. I'm not sure if the grape vines can be automated though, so I think it is a good trade-off.