It all depends on what you're looking for.
If you are thinking horsepower, the only thing you can do to make a big difference is add a turbo. I don't think Scion/Toyota make one anymore so that would have to be aftermarket (though I think the Scion was just a rebranded aftermarket).
I added a TRD rear sway bar. I think it made a huge difference in decreasing body roll around corners. Many others agree. You can go aftermarket with Hotchkiss though and I think you get a front and rear for about the price of the TRD rear only. The a rear sway bar installs in a few minutes. The fronts are much more labor intensive. Tartan Jack seems to love his Hotchkiss set, but he does some fast cornering on his "mountain twisties".
Springs are a common mod. The real low springs like the DF210's aren't going to be for performance. Less low like TRD's I think are low more to lower the center of gravity for better handling. (someone please correct me if I'm wrong there) The NF210's would probably fall in the same category as they are also progressive springs and have a similar drop to the TRD's.
From the many threads I've read a front strut brace can help handling if you take corners VERY aggressively. Otherwise it is mostly for looks. The TRD one is the only strut brace I've seen that is a solid piece. The eBay ones have bolts on the ends. There seems to be disagreement in the xB community as to whether that "flex" point is detrimental or not.
Then there is the whole debate about intakes (cai vs. short ram). It seems the consensus is that it doesn't actually add much hp. It does make it sound mean though.
I think I saw a dyno output graph that showed a cai actually took a little hp off the lower rpms but added a few hp to the midrange. As it is basically a straight metal pipe, I don't see how one brand would be much better than another as long as it is smooth and doesn't leak. Many people have praised their cheap eBay cold air intakes, but at least one guy said he smoothed out some rough welds on his.
Of course there are also all of the exhaust mods (headers, scuba deletes, axle-backs, etc.) As with the intakes, I think it can add a few hp but no huge gains. I think a big reason is sound. That's all I expected from my Borla dual axle-back. I believe the TRD exhaust is also an axle-back design. Each brand might sound/perform a little differently depending on design, pipe diameter, etc. For just an axle-back though I don't see how they could be very different other than single/dual.
Ok, so wow I kind of went crazy there. Sorry for the huge post. I hope you didn't fall asleep in the middle of it. Keep in mind that I'm not a mechanic and most of what I know is from reading everything I could on several forums over the years. Please anyone feel free to call BS or correct any errors or misstatements. :cheers:
Hopefully I answered your question. I feel like I went off topic a bit there.