I'm sorry but it sounds like he's trying to bullshit you into buying his product. And he does have a reason why 421 is better for Alza and not Myvi. First of all 421 or 41 need to be tuned length, not just any how slap on a 421 and let it be. Need to calculate the length of each runner to get the optimum exhaust flow. Do a Google on how to calculate exhaust length.
And Alza obviously is longer therefore easier to do a tuned length 421. Myvi on the other hand is way too short (wheelbase). But to say it's only meant for 41 is just his word against someone elses word.
The thing is myvi and alza are using the same 41 and 421 extractor design...so the run is the same. So if it works in Alza, no way it wouldnt work in myvi.


