Here's another worthy response since I'm such a detailed guy. LOL

*As far as stainless in thinner gauge for such a crucial part member, Yes, I would be a little high nerved on a high strung wheel standing car, especially a heavy car. But on a decent HP car that sees hardly a wheel stand on launches like mine, then yes I would try it while keeping a close eye on its stress points.
In building one, 440 Stainless would be my choice where as 304 stainless is my second. Back then, it was different stainless quality metals that they tried. Even a few or so Magnesium samples were made. Crazy.

I don't rag and trash my car while driving under normal conditions, I see to it to save it for the runs.

Why?

A friend of mine here in Brooklyn way long ago had one of the 65' stainless units on his heavy 3750 LB Plymouth Hemi street race car and didn't have any problems till he started with the wheel stands. Shortly, it showed signs of cracking. He threw in the garbage years later and way before I got to hang with him enough to talk about it.

A thin gauge steel member might not survive as well. Stainless to my knowledge gets its strength from not only the temping, but also the chromium that is part of its properties.
I'm honestly not at all a well tuned metallurgist here.

*As far as what I have seen, experienced, tested and so on.......
Those aluminum seat brackets have gussets introduced to the inner sides. Like mentioned before they have served on my car for a long time with no cracking issues what soever from regular service. They are strong but not as strong if they were steel and if it were so, it would defeat the purpose.
Then again, plastic racing seats are only as strong as their impregnated steel gussets and bolt bosses. The plastic could shred and or crack wide open in a bad crash, so I think it is irrelevant.

*Yes, part of my original calculation of using a 3" exhaust has to do with heat expansion. Probably would keep more heat in the system along with torque in the curve if it were 2.5 ID, but I wanted to make sure I did not have aluminum soup on the street.

* The white cross member is aluminum and gusseted in every way possible using T-6 grade.

*6AL-4V T titanium which is in question here can hang in stress points as much as steel can. But probably not as strong as high tensile steel bolts. It also has high anti-corrosion properties unlike steel.
Used in many high stress military, marine and aircraft/aerospace industry internal engines and turbines where there is a lot of stress and heat.
Also consider that there are (5) 6AL-4V T studs holding each wheel onto the hub. 7075-T Aluminum hubs by the way.

*Now thinking about it, Lightweight Axle U-bolts can take a vacation as far as I'm concerned only because a good hooking car puts a lot of stress on those bolts as the rear tries to twist itself around on the hit. The leafs, driveshaft, gears climbing on each other along with the weight of the car are all calling on those bolts.

*I would never play with hollowing out TB's (Torsion Bars)since they hold most of the front weight up in the air and take lots of load under bouncing (Street use) or landing (wheel stands).