For a base (non-PP) car, is there any benefit to adding a strut tower brace by itself, without the cowl bracing? I get that it would all work best with the cowl bracing to tie it together, but interested to hear any opinions.
Fills a gap and stabilizes the mounting point at the firewall. There are others on here that said they felt a handling improvement after they installed it later.
I did cowl bracing first. It’s far more noticeable in day to day. Eliminates some front end creaks over bumps you didn’t realize were there too. STB made less of an obvious difference for a much greater cost.
STB is for looks mostly but anyone that installed will tell you that it absolutely makes a difference. If you want it, put it on. It does make engine bay look better but access to dipstick (on a GT at least) is harder.
I added the brace back to my PP. I bought it, and it was delivered without the strut tower brace (used). I bought the standard one and added it back, but I don't think that I can really tell the difference. It looks kind of cool, but it kind of gets in the way, so the first instruction of anything you do to the top end is going to be "remove the 4 nuts holding the brace on and set it aside..."
IMO the cowl brace is more important. Think of it as a bent STB. It will still stiffen the two shock towers with each other side-to-side, but it will also stiffen them front-to-back.
If you add the cowl brace, be sure to also install the support bracket with it. (The bolt location at the firewall is just thin sheet metal without the steel support bracket.)