This might be a stupid question, but how does the Legacy B4 stack up to the STi?

At STi is faster but not by alot and an RSKB4 will still beat almost everything else on the road.

An STi has more potential for mods as it has much stronger internals and a single turbo.

I have never heard of anybody doing big mods on twin turbo legacies. The most I have seen people do are muffler (not even entire exhaust), intake and Link ECU...