a shift kit? is this atx or mtx.. afaik - theres no shift kit for our cars
its either a> the escuse one uses when it does the hard into 2nd shift trick, or b> the IAT mod, which forces it not to cut off the fuel in between shifts (luxury shift mode) and can do a good deal for your 1/4 mile times.
fuel filter another excellent idea..
cap and rotor are basic tune up items
if the plugs were old, the wires may be as well.. check them out, make sure no cracks in the boots, etc...
run a bottle of fuel injector cleaner.. chevron...
make sure your VRIS is working, do the paper trick (pm me for details if its hard to understand from the posts... but youre a handy guy, you'll figure it out i'm sure)
thats pretty much it... without getting into more extensive stuff