Mex, yes 1/8" is worth it. What you should do is go about 6' forward from the transom, since this is probably where your boat rides. Since you didn't say what kind of boat or bottom you are dealing with, I'll assume you are talking about a shallow V. You should go to your outermost lifting strake. Fill gaps by grinding through the gel & barely into the glass with some 80 grit. fill the large gaps with fiberglass cloth until you get it to the point where you can fill to smooth & finish straightening with bondo with stranded glass in it. You don't want the running surface of your boat slick. Leave it rough, as this traps little bubbles of air-almost like tiny ball bearings. I left 80 grit scratches in mine. Speedcoat probably isn't going to make a bit of difference on your boat. Most use it because it is easy to cover bottom repairs, and gives your boat sort of a "stealth bottom" look. If you were racing a tunnel hull, and looking for that last 100th of a second...then speedcoat might be worth it. I seriously doubt that you will ever be able to tell it's on there or not.
Good luck with your project!
Ken F