So would you say that using smaller pots (6-7") for ring culture in growbags would have a negative effect on growth compared to just growbags?
Well I have never ever done this myself but my guess is the top pot might become pot bound which could prevent water/food penetration to the feeding roots.
What could happen if this does occur that the potted part of the root system might draw up nutrients by capillary attraction yet remain relatively dry, but this is not guaranteed :(
This might also create ideal conditions for 'blossom end rot' due to irregular watering!
Can I suggest you insert a yogurt pot or something similar into your pot now before it fills with roots as seen in the picture below.
If the worst comes to the worst you can remove this carton to leave a hole through the root system when the pot becomes 'pot bound' that you can fill with water/feeding that will get the moisture to where it is needed.
As I say I have never come across such an event but I think what I suggest might be useful insurance in the event of such a problem.
If the situation doesn't occur then thats fine!
I would like to hear at the end of the growing season as to how you got on,
Who knows you might have invented a new workablr technique! ;)