An offset bushing in the stock bracket is ST_ legal. There is no allowence for a new bracket that moves the bushing from the original location. It's in The Solo rules 14.8 C.
The ALK removes some of the anti dive geometry in the stock suspension. If you read those two books I mention in my reply, what happens is anti dive geometry keeps the nose dropping with soft springs by locking up to an extent. With stiffer springs you don't need this as much and can gain more compliance and traction at corner entry.
There is just not enough room to do all of this inside the stock diameter bushing with a two axis ofset so max castor is what the offset bushings go for.
I too fell into the trap thinking it was "only" a castor change, but the ALK is a way to remove the anti dive geometry.
The STi does have more castor than a WRX. If you look at the wheelbase it's longer. Subaru shoved the front hubs forward 0.6". With that said, I've been thinking that rotating the bushings 90 deg in STU might be more of an advantage. Who knows. I just run AS.