Other than pistons all the parts are interchangeable so it ultimately it just comes down to bore size. 3.91 bore is big enough to make the hp the average dude wants, not saying a 4" or 4.04" plus wouldn't better or a 4.185"-4.34" plus isn't even better.
Most of this 318 can't strawman argument is ridiculous, I'd say the vast majority of people building a 318 just wants a mild hop up, usually just wanting to add cam 4bbl headers to a running engine. To act like these people are wanting to do something that's bordering on the impossible is insane and for the few wanting to build a 318 full of aftermarket goodies it's ridiculous to think it can't perform. Is it's worthwhile that's for the OP's to decide.