Here's a couple of tricks I found that I didn't use in either my any% or min boxes runs. Note the min box run has a useful strat for any% that I didn't use in my any% run, which is skipping a box in the room right after "grab the bootstraps".

The first is in the first room with two boxes. This is the hardest trick I've found for show, requiring two boosted jumps and accurate timing on the box jump. It isn't necessary to land on the box the way I did here, as you can do a superjump out of a box jump, but I failed it enough times that I wanted to be careful.

The second is in the first room with a key, skipping the key. I'm not sure which is faster in this room—skipping the key or skipping the box. Skipping the key saves you the time doubling back, but if you skip the box you can get much further to the right before the level loads.

New strat that I believe was first confirmed by @Crayonatee, makes an 8-box run possible and should save lots of time in "the impossible""