I have the Boomba kit (shifter and bushings and under hood piece) and mustang knob. It feels very good. I come from acuras and toyotas with super short shifters (short meaning both throw and tallness) to the fiesta but it's really fine. Keep in mind for every little bit you lower the knob you are shortening the lever and therefore increasing the effort needed to row through the gears.
The Boomba lever adds a nice angle. If you have long arms like me you might think about an armrest cover/pad because your elbow will hit the armrest and be uncomfortable. With the pad mine is perrrrrfecto.
I was skeptical of all these things before buying but I actually really like my setup now, don't know that shortening the height any more would actually be a good thing. And I'll add that doing shifter mods on our cars is very easy, much easier than in the Japanese cars I've done the same to.