I really like the Bilstein dampers I have on another car, but when I look at B16 coilovers for the Subaru there is a notation that the rears require 15mm spacers. That's really disappointing. I'd not do that.
On the Öhlins website, the Subaru Impreza applications listed are much younger than the GD.
You need to ask yourself what you value in your suspension. Performance, comfort, reliability, fitment, quality, or lol coils brah.
The bc falls at the end. For absolute fact rce springs and koni yellow inserts will obliterate any cheap coilovers on the market in every metric besides lol coils brah. I would suggest looking no further unless you have money to spend.
Inexpensive coilovers rarely have the travel, the dampening, or the spring rate to make a good ride, or a well performing suspension. It isn't as easy as 66000000 way adjustable. You've also got quality aspects like short usable life before rebuild, unstable ride height, and in some cases weak parts.
When i had to redo my suspension I looked long and hard at coilover options. The cheapest recommended to me by a shop I trust was feal 441, these came at the cost of unpleasant ride on the road. The shop offering them valved and springed them for my car specifically. Ultimately i opted for factory suspension as my driving skill is not honed enough to take me outside of factory suspension capabilities.
If you are looking to tighten things up consider affordable options like bushings and swaybars first.