915 heads vs 302 heads

The chambers on the 915's are larger meaning all else being equal you'll lose some compression. The ports themselves are also bigger and flow more, but a larger port can hurt low RPM throttle response. Larger intake valves will make a cam seem "larger" which could be good or bad depending on your current setup.

Assuming you are running a 318 now, the 302 heads will support 275-325 HP depending what's been done to them. If you are actually making over 300 HP now, the bigger heads would be a worthwhile swap, otherwise you probably won't gain much.