You probably need to decrease caster as that will help with the steering effort at low speeds, iirc, non power steering Miata's are supposed to have the caster at around 4.5 degrees while power steering Miata's are supposed to be at around 5.5 degrees.
Yup...more information here:
"Caster
More caster can help mid to low speed turn in, makes steering heavier, may increase wheelspin in very low speed turns with a Torsen or open type differential. Higher caster values will increase stering effort. Lower values reduce steering effort. If running power steering, go for caster settings above 3.5°. If running manual or depowered rack, lower caster can reduce steering effort. The Miata platform likes caster greater than 3.5°. Balance that against the steering effort you prefer. Running caster beloow about 2° can result in very vage steering with little or no self centering force."
http://949racing.com/miata-race-alignment-info.aspx