'96 jetta cold air intake kit

regarding after market performance boosters , I am looking for references to reliable technical literature describing the real versus advertised effects of cold air intake kit. (as well as "cat. back performance muffler kits")
My son swears that his 96 jetta 2.0 liter standard has more power now. My mechanic says there is no way to make appreciable gains in power when the throat size of the catalytic remains the same size. AND we have some concern that the cold air intake may start to affect the $480 air mass/flow meter.
Plus, it may be a coincidence, but the car died two or three days after the cold air intake was installed. Intake was installed a day after we got car back from shop with new/used ECM and used distributor.(that is discussed in earlier thread about crank sensor)
Any relevant info appreciated.Thanks all.

NEGITIVE INFO... I have a 97 vw jetta 2.0L. I have a computer chip,underdrive pulley, big bore intake tube, cam, gear, header and cat back exhaust with servral other things.. i have 106,000 miles. really has been a good car. I do 90% of the work on it at my work.. Noticed increase w cat back!!! really speeded up with the header.. i also have the k&N air filter with heat shield.. actually works well...In my opinion I LOVE MY CAR.. normal maint. req.. i think there may have been a problem wit the ECM or other parts.. I wouldn't EVER buy a used computer..