Assuming stock parts on a 302 and the same stock parts on a 306, barely anything. The difference is gonna come with aftermarket parts, compression ratios and/or power adders on a 306 vs. a 302.
very little when i put my new 306 in i used all the same parts that were on the stock short block i picked up 5 hp and i think it was due to the bigger tb maf and 24# injectors over the increase in size
I had a guy up in jacksonville tell me something about when you bore it out, it unshrouds the valves, and does something for airflow. I think really the only way to tell the difference would be to build a brand new 302, then bore it and build it again.. which no one is ever going to do.. Lol... My before and after dyno's on my original motor had Awesome gains going from a 302 to a 306 changing nothing, but then again... the old motor had 78,000 HARD miles on it. Lol... I dont think we will ever know.