From what i heard, not that much, maybe like 30 HP from just the shortblock. Mostly due to the better blue printing and balancing. I heard the big bores are the way to go, especially with boost. However this is only what i heard, im still running a regular 4.6. Oh but for the price of the CHP stroker it is very tempting.
you will gain some hp from the extra bore but you will lose that power because of the stroke. on average if you swap straight over , short block to short block, will will gain about 10hp at the wheels and about 30lbs of torque. The numbers are not set in stone but that is what I have seen from a lot of dyno sheets and thins like that.
ya the stroker 4.6 is a bad idea unless its with a big bore(3.700" bore). the 4.6 respends better to a big bore because it unshrouds the valves and allows the air to flow into the combustion chamber more freely. the stroke is already long enough on the stock motor (3.55") and you dont want to make the block oversquare like on the 5.4.
if you really want to make a bigger 4.6, just get a big bore. to do that you will have to get your block bored out a lot, then resleeved.