Having held and looked over Tein, F/F, Megan, ksport, yonaka, and a few others I can definitely say Tein is a step above all the other Taiwanese made options. Tein still puts out a better product than all those marketing hype brands. I've ridden in tons of cars with cheap-os and they're generally the same.
I've also owned F/F and Teins and the Tein's are hands down a higher quality part. Perform better as well. Tein does actually spend money on R&D too. Teins may not be as quality of a part as KW or Eibach, but they aren't on the same level as all the cheap stance coils.
One brand I would like to check out at some point is Fortune Auto - they have a ton of spring options including Swift springs and I've heard alot of positive feedback from them.