There are a number of things at work here.

First - when you narrowed the exit you increased the efflux (ehaust speed of the air) which is really everything. Narrowing the opening will increase top speed (at the expense of static thrust) and opening it up will increase static thrust (at the expense of top speed) You may have to experiment to fid the optimum opening to get what you want.

Also - just like a prop - different fans have different pitch. Higher pitch will result in higher top speed but longer takeoffs.