Would you happen to know the specs on that cam? Is that on the stock ecm tune or custom tune? I've haven't personally seen one on a dyno but a lot of people say a ecm tune alone is worth about 50 hp. It is worth noting that the injectors will probably run out of flow above 350 hp, so if you were going through the work of a cam swap it'd be worth upgrading injectors and having the ecm tuned appropriately.
Here is a quote form our conversation;
I do a stump puller for these. It is 206/220 .470/.420 lift on a 109. Makes around 520#/ft and 350HP.
I asked specifically if this set up would work with the factory tune and FI, he commented that he had installed this cam in many of the Mercury EFI units with no tuning required. I also asked about the stock heads and valve train, he commented that while not optimal it will work without issue.
Neal