vtec is never a hinderance when making power.

it all depends on tuning.

for boost, you want the best flowing setup available. while flow isn't critical it definitely helps when making power.

to say the a6 head is better for boost over 20psi doesn't really make any sense either. stock vs stock the better head will be one that can flow more air. in this case, its the vtec head as honda designed it to make more horsepower on the high lobe and the rest of the head is designed accordingly.