I was watching demolition14's world record to see where I could save time. I noticed that they always got rid of allies when they didn't need them. So I was wondering why? Do you do more damage without them, resulting in quicker boss fights?

Yes

I think the bosses just have less HP with only 1 Player. It's common to have boss health scaling like that in multiplayer games.

I decided to go and time me fighting whispy woods using only the upward bluster sword slash and only blade knight allies. I fought him 10 times (to make sure the results were accurate) with 3 allies + kirby, 2 allies +kirby, 1 allie + kirby and just kirby (so a total of 40 fights)

3 allies + kirby = 24.3 seconds 2 allies + kirby = 23.2 seconds 1 allie + kirby = 22.2 seconds kirby = 19.1 seconds

Kirby on his own is a lot faster however the difference between allies is very minor only about a second

OH DIP. No wonder my ultimate choice runs have been so varied