I have done quite some research about equal/unequal, since non-turbos have equal, therefore arent sounding as nice.
There is a myth, that the power/torque can be affected. Usually this is caused by improper or faulty installation, where, according to NASIOC, the backpressure isnt kept the same when changing the headers. That could result in loss of up to 5hp when installing UEL instead of equal lenght.
Honestly? 5HP is a price I am willing to pay to get the sound that makes subarus subaru. Even though, my car sounds great even without UEL's its just that little thing, that will make it sound purrrrrfect
[youtube]LqNvMPiCLsU?t=46s[/youtube]
2015 WRX has FA Engine and from factory installed Equal lenght headers, which can however be replaced with for example BORLA or any other system, there will be more coming soon. Subaru decided to go the quieter option for more reasons, one of them being, trying to bring WRX more into mainstream, especially in US.