I just broke a coil on my Legacy GT 1998 and I need one now!! It's more easy and cheaper to get four used Impreza RS and WRX coils than only one OEM. So, I want to know if Impreza RS of WRX coils will fit my car. Even if it's not for an upgrade, I don't have any problem with higher spring rate!
I would highly advise against running mix matched coil springs. If you were to run all RS springs or all WRX springs that would "ok" but you wouldn't want to mix your legacy springs with either of the others.
The RS springs will probably fit on your legacy, however the spring rates will probably be lower since the RS is a lighter car, and handling will probably be worse. Not the best upgrade IMO.....