Yes, a larger rear sway bar will help reduce under-steer TENDENCY.
Yes, stock RSB is 16 mm.
Yes, if you replaced the front bar with a 25 mm Tribeca bar you will increase TENDENCY. The tendency would be much more prone to under-steer than stock.
I say tendency, because any car, properly driven to (and only to, and not beyond) it's limits will take any turn neutrally.
Yes, an 18mm OB RSB is perfectly fine to use. The difference is almost indescribable though. 2 mm on a hollow sway bar isn't enough to really notice. Now, if you are going to install it, make sure that you have the frame/chassis mount as you won't be able to install it otherwise.
Using a 25 mm hollow Tribeca FSB with an 18 mm OB RSB will still increase the tendency to under-steer. Why? The ratio is still biased. You'd then have a FSB that is 7 mm larger up front than the rear. Stock is 20 mm up front, so that's only a 5 mm difference. Will you notice the difference? Probably not, but body motions may be held in check a bit more than stock.
I would hold off on installing any sway bars until you can install both the hollow Tribeca 25 mm FSB and the solid 22mm RSB.
Agreed. Upgrade the end-links is very necessary.
This setup is more balanced, because the 25 mm hollow is nearer to the strength (if not maybe a bit less) of the 22m solid bar.
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Remember, any car can be driven with a neutral balance. Changing the balance will only change how you need to control it, as well as change under/over-steer tendencies. On dry pavement, not being able to rotate a stock sedan as easily (if at all, really) as a stock wagon is annoying as hell!!!
Speaking of which, do you have a sedan or a wagon?