when you use the same pads(koolstop salmon for everything here in the PNW) there is no determinable difference between them that can't be accounted for in terms of differences in lever feel. The most important thing is to just match cable pull between the calipers and levers. I've mismatched shimano SLR and SLR-EV and you either get too much lever travel or too little. Major issues if you have smaller hands and use the shims or adjustment so you are limited in how much travel you have to work with.

this doesn't tell you anything except that there were differences in setup, pad material, rim width etc between the bikes in addition to the differences in brands of calipers and levers

Why is this so hard to understand?

You squeeze the brake lever.

For whatever reason it feels either spongy or firm.

I prefer firm...

Because it doesn't actually tell you anything. Firm 3mm into squeezing? 10mm? Squishy immediately upon grip or after 10mm? Same pads? same housing? Same cables? Going into a bike shop and squeezing brakes that you didn't set up doesn't tell you anything about caliper stiffness at all.

Yes. I have eTap with ULTEGRA BR6800 (now upgraded to DURA-ACE BR9000 + I'm also using a DURA-ACE cassette 11-28).

Be careful replacing your TEKTRO because it might be the "long reach" model (R580). You might end up needing an offset holder. I made that mistake on my Synapse.

It tells me how they feel; what else do you think I need to know?

Why is everyone so analytical about this?

It's just about feel.

And, if you look at the caliper while squeezing the lever you can see how much it flexes and even compare it to others.
Feel free to bring your micrometer if you wish...