Newbie 318 piston question

why do people still go on about valve shrouding using bigger valves/ 340/360/aftermarket heads on a 318? the bore is only 90 thou (stock) smaller than the 'majic' 4" bore of a 360 which is only 45 thou (spark plug gap for reference) closer to the inlet valve. even with the added compression drop of fitting 340/360 heads to an already low compression 318 it's been proven many times they make more power. so what's the problem?
318 bashing maybe? oops, did i say that out loud? :rofl:
neil.