Allowing an engine to breathe better will ALWAYS increase performance. It doesn't matter if it's been bored or stroked or whatever. Of course if you took a 420cc stock engine and added an exhaust and high flow filter to it, it's not going to respond the same as a 420 engine that has been punched out to say, 550cc. But the fact remains that there is a 10-15% power increase when adding an aftermarket pipe and tuner to a STOCK engine. No other mods. Once you start doing big bore kits and stroker cranks you'll find that there is airflow restrictions elsewhere, and you'll see guys doing head porting, bigger valves, larger throttle bodies, custom airboxes...stuff that really wouldn't be needed on a stock engine with a few bolt ons.
As far as cams go like I said, there isn't much for the 420/500. Nobody has one off the shelf, Griffin Performance does if you don't mind buying parts from a convicted pedophile.....I think Hormell's may have one but I have no idea whether you have to buy an entire package from them to get it or what's the case, if the price on that is anything like their GR prices it's probably ridiculous too. Also if your going to replace the cam you might as well do at least a 500 top end at the same time, because these pushrod engines need to have the head removed to change the cam. So suddenly you go from buying a pipe and tuner for $400~ to buying an entire assortment of performance parts for $1500+ and add to that a TON of extra labour, not even going to get into the cost of a port and polish job or any other custom stuff.
Yes no replacement for displacement, but there is plenty of cases where guys can make MORE power with a stock bore engine than other guys will with a big bore or stroker just because there is more to it than just slapping a bigger piston in it and calling it a day.