it's there and it's not. you're going to have to edit the keybind to do it yourself.
the command is throttleadjust. simply put in your chatbox [/bind e throttleadjust .1] minus the brackets  to make the trottle adjust 10% every time you push e. to decrease, you would do [/bind q throttleadjust -.1] again minus the brackets.
for 5%, it's .05 and so on.
caveat emptor: i'd stick with the 10%. otherwise, it becomes more of a pain to micromanage your speed settings.
Another thing you can do with keybinds is to throw throttle on your mouse wheel, its makes it easier to adjust quickly with the finer increments and more importantly frees up q and e for keybinds you want quick access to. Also, you can save out all your current binds using "/bind_save_file filename", edit the file and load it back in using "/bind_load_file filename".