Okay. 600F is still way the hell more preheated than 80F air. By "consumable," just how "consumable" are we talking here? Would a 1200 degree pipe melt in one firing? Or do you just mean that it won't last a lifetime? Because a couple of 90 degree iron elbows and straight sections cost only a few dollars. Even if I have to replace it once every one or two firings, it seems cost efficient if it halves my fuel usage, for example.
Ideally what I'd want is some graph with "temperature exposed" on the X axis and "number of heating cycles until loss of integrity" on the Y axis for common iron pipe, or something like that. Then I could use the cost of the pipe versus the cost of wood versus that graph to pick a precise maximum efficiency point for me. Then test different lengths of pipe extending further into the chimney until the output temperature is measured at the desired level. Maybe no such thing exists due to it being more complex than that, though.
But anyway, yeah it doesn't have to be fully up to temperature of the kiln -- any amount closer = fuel saved. Just want to try and estimate how close I can push it before investing in all the other costs of the kiln.
I think you may need to do some deciding about what's most important to you in this project. It seems to me that you need to decide if making your design work is more important or making a fuel efficient or cost effective design is more important.
Here's where I'm coming from: 600F is indeed much hotter than ambient air temp, but I think you'll find it difficult to make a draft work if you run your air intake only part way down the chimney. There's the question of what you're going to do for the rest of the air intake. If you run steel all they way down the chimney your steel/iron components will be very consumable indeed. Not only is temperature a factor, but also the fact that corrosive gases like sulphur dioxide and hydrochloric acid will be off-gassed during firing, especially if your clay content has a high amount of organic material. Based on your prior posts, it appears that it will. Hot steel plus corrosive gases equals decay. You may be able to get one firing out of them before your heat exchanger starts spitting iron oxides at your pots. In all likelihood, if you're approaching cone 08-06 on your first firing, you'll be blowing FeO flakes into your ware right then and there--that's the temperature range where FeO begins to flake off on its own. No good if you don't want little black/red stains on your bisque or FeO chips in your glaze.
If you're committed to the design, you will need to do quite a bit of R&D, which is expensive and time consuming. There's likely something there, though, if you want to figure it out. The X/Y graph data you seek, I don't think exists, and if it does, I don't know where to find it. The materials science department at your local university might be able to help. In all likelihood, it will be a part of the research you have to do yourself.
If you're committed to keeping fuel costs low, trade wood out for any other source of heat. Wood/charcoal is the most expensive, least effective way of getting your pots fired. I think even those who fire with wood will agree to that. Waste oil burners are easy to make, and the fuel can be had for a smile and a song. You could certainly cobble together a waste oil burning kiln that would take you up to cone 10 very cheaply. You can get used fire bricks for the cost of red clay pavers from some brick recycling places, if you're a good negotiator. There's a place I know around my area that reclaims fire brick for the alumina, I got about 20 bricks from them for nearly nothing for a project I was working on. Those with a few waste oil burners, some angle iron, and a little knowledge, would give you a kiln most potters would be envious of. Would cost probably 5% of your wood/charcoal budget to run it, too.
I don't know, something to look into. I know a guy who's very much like you, I think. He's got a tremendous amount of talent and imagination, but gets hung up on reinventing the wheel. His latest endeavours have been to bring serious drama back to puppetry and become music's next "musician's musician." I didn't have the heart to tell him about the existence of Jim Henson's creature shop and what they do. Forgive me if this is a little too personal, impertinent, or isn't relevant, but this guy's been on my mind a lot lately--I'm more than a little worried about him. I wouldn't want you to get hung up in the same mental traps. Again, apologies for any perceived impertinence.