Removing the restictive inner walls in the upper plenum will give you how much of a performance gain? Is it noticeable and worth the trouble? How did it feel up top after?

 

On my 305, I didn't notice too much in the lower rpm's but the motor pulled a lot harder in the upper rpm range. I also did some porting around the runner holes. There's a "wall" between the sets of holes that you can remove some material from.

 

My setup is in my sig . With the whole aftermarkeet TPI base and runners ported AND the plenum . I think you need it all done at once to notice it . I don't see a huge gain from it unless you incorporate other intake mods to exaggerate it's benifet. This is mine
With the runners ported and this done . It just made the power curve fall of more gradually past the 4800 rpm mark. I still ended up looseing only 15 hp from 4800 to 5400. So I say it does help...a bit