I'm thinking about picking this up during the current Steam Sale, mainly with the purpose of speedrunning it. I very much prefer playing games with keyboard over a controller. I know this game has limited aiming when using keyboard, my question is does this affect the speedrun much, or can you still speedrun this fine using keyboard?

In my opinion, speedrunning it with only a keyboard is possible, you'll just need to train more I think, to be get used to it.
For example, Louffy, a street fighter player, is, as far as I know, the only player playing with a pad and he won one of the greatest competitions of SF. So I don't believe the tool is not important, this is the training that you'll do C:

There are a few bosses that are quite a bit harder w/o controller (For instance, Soul. You need to basically get lucky w/ keyboard, where you can actually aim and hit him consistently as long as you have good reflexes w/ a controller)

For posterity: We now have reliable quick kill strats for keyboard runners. Controller is still advantageous especially for movement.