1. I am guessing this only speeds up the daylight cycle right? It doesn't modify things like making crops grow faster or furnaces smelt faster, right?
2. I'd say it should reduce mob spawns in your local area. So if Players A and B are 2k blocks apart and Player A sleeps, mobs won't spawn around them at all and the overall mob cap gets reduced as you mentioned. But mobs would still spawn by Player B. That way Player A is more guaranteed to be safe while waiting through the night (assuming their bed isn't 100% secured), and Player B isn't completely screwed out of spending the night hunting mobs, even if that night is shorter.