With my 2.5RS I wanted to do the blue interior with JDM V7 Alcantara seats. The JDM seats are much more comfortable than the USDM seats. The problem is that blue carpet was only ever available for 4 door Subaru's. It was not meant to fit in a coupe.
I'm sure there are aftermarket carpet companies that can make "molded" carpet in whatever color you want. But I've tried aftermarket carpet for another platform and I was not impressed with fitment. I had seen pictures of blue 4 door STI carpet in a GC coupe before, and it seemed to fit well enough for me to give it a try. So about 10 years ago when I had my first GC that is exactly what I did. I installed it as-is and lived with it. It wasn't perfect, but it wasn't awful either. I've had several GC's since and I've installed STI carpet in just about every one of them. Each time I do, I seem to find ways of doing things differently. I've got it down to a science now and decided to share that.
Here is a video of me explaining:
So aside from the obvious stuff to make it fit, there are really 4 additional tricks I used. The video probably explains it better than I can in words, but I'll try my best regardless:)
Trick 1: If you are installing GD rear seats, cut two small sections about 12" x 12" out of your oem carpet (wherever it's the nicest looking). This can be placed on the sides of the rear seat under the cushion to help cover the gaps. The GD seat starts to curve on the side edges, so if you look down the sides of the bottom cushion you will see exposed chassis.
Trick 2: The rear edge of the STI carpet is shorter as there are cutouts for the middle B pillar that would be on a 4 door. This edge is what you try to tuck under the door sill. Because the material is shorter in this area it doesn't like to stay tucked under, especially if someone sits in the back. They will step into the rear of the car which will tug the carpet downward. I added 4-5 layers of Fatmat (alternative to Dynamat) underneath the carpet in the rear corners. This helps hold the carpet higher where it won't tug out even if someone steps on it.
Trick 3: The trunk release lever on a GD sits further ahead than the GC. So when you install the carpet you are going to have a square hole right in front of the trunk release lever. You'd also have to cut some of the carpet on the back side as it would interfere with the GC lever assembly. Instead, do not cut anything. Simply remove the lever assembly and drill a new hole in it so that it can move forward to reuse the same GD hole.
Trick 4: The seat belts of a GD anchor into the car in a different spot than the GC. So you will have some extra holes in the carpet. In addition the carpet bulges in this area to make room for the belt to pass through. It's not ideal and I have lived with this for years... until now. You can cut start by cutting the bulged area down the center. It will then lay flat if you overlap one side. You can carefully cut a pie shaped piece out of one side so that it will lay flat with no overlap. Basically both sides of the pre-existing bulge will meet in the center. Then you can cut the oval seat belt hole out in a rectangular fashion (It will be easier to cut a square patch than an oval one). From there you can steal an extra piece of carpet from a non visible area of the car (I used a section that the rear seat cushion would cover) and cut it to fit. Use some spray adhesive and non-stretchy fabric on the back side of the carpet and press everything together. Here are some pics of the process.