It's like most things, you 'rob Peter to pay Paul', so between these two it's efficiency Vs BW gain. The 515 has a weak bottom end compared to the 416, though I imagine the main reason they weren't used in HI-Fi apps was their higher cost, but EQ them both flat and compare them at the same amplitude over the same BW that they were designed for and they will sound the same, though there will be some folks who'll swear on a stack of bibles they can hear the difference of the 515's ~5% lower Mms.

What folks do hear though since they typically don't EQ the 515 flat is its greater rising response that when used with a typical Altec textbook XO causes a hump in the FR above ~500Hz, ergo it sounds 'faster'/'higher resolution'/etc.. It's this 'quality' that makes it the better driver (whether horn loaded or not) for blending to the big multicells and sectoral horns that are coupled to the higher efficiency drivers.

Bottom line, unless you either use dual 515s/channel or a highly damped sub system of sufficient efficiency, then IMO the 416 is the choice for all Altec based Hi-Fi apps since too much EQ/eff. loss is required to flatten the 515.

GM