Your hesitation could be caused by many things. A vacuum leak, dirty injectors, old spark plugs or wires, dirty fuel pump filter, etc. Suggest giving the engine a good tune up and chack all the vacuum lines.
As for cold air intake, The Fiero came from the GM factory with a cold air intake. The intake is the vent on the driver side just behind the door. If you put on a cone filter with a chrome tube, You will have a hot air intake and power will go down. Most CAI's use the front of the car for the cold air. You cannot do it wit a Fiero because it is mid-engined. Hence why Pontiac gave Fiero's CAI's.
For cost efficient upgrades, The best one that costs very little money is to port the exhaust manifolds. Pop one off and you will notice that the holes in the ports are oval and restrictive. You can port them out and re-weld the bead on both of them and gain 5-8hp.
The Fiero Store sells a larger throttlebody. The TB and the exhaust porting together will give you 10-12hp gain.