I actually tested this a while back.
Each world interaction upgrade produces one cobblestone each time the node triggers its "make cobblestone" function, which occurs once per second (and cannot be accelerated with speed upgrades). So, one stack of world interaction upgrades will produce one stack of cobblestone per second; two will produce two stacks; etc. However, any cobblestone beyond one stack will go into a hidden buffer within the transfer node, from which it cannot be immediately removed. The node can only do anything with the visible buffer that everyone's familiar with, which only holds one stack. In order to get anything out of the hidden buffer, you have to clear out the visible buffer first, at which point it will instantaneously be refilled from the hidden buffer (assuming there was anything there). So, to get the most out of your transfer node, you will want to give it five stacks of world interaction upgrades (for five stacks of cobblestone per second) and enough speed upgrades for it to path to whatever you're pumping cobblestone into five times over every second.
You can verify this by having your transfer node pump cobblestone into something with a very large inventory (such as a diamond chest or a barrel), turning it off with a redstone signal, putting in five stacks of world interaction upgrades and one stack of speed upgrades, turning it on, and looking at whatever inventory you hooked it up to. Every second, you'll see five stacks of cobblestone appear in quick succession.
Of course, you won't need a full stack of speed upgrades if you're only transporting the cobblestone a short distance. I don't know the formula for that off the top of my head; fiddle with it, see what works. If your cobblegen pauses every second or so while running, you have as many speed upgrades as you need.