[1999 White C5 Coupe]

Is it the newer body style?... check the 8th gen forums with the LFX engine. Few guys have full bolt ons.
Personally I felt the biggest upgrade to be the Overkill tune, better engine responce everywhere, better shifting.
I have full straight pipes from the downpipes back, everytime i cut off something (cat, res, muffler) i lost low end torque, but seem to have gained a decent bit around 4k-7k.
Also the Overkill ASI Intake helped the mid/top range, didnt seem to effect low end.
Their isnt really much for our 9th gens lol.
Intake, 80mm overkill tb, jacfab 3/4 inch intake manifold spacer, magnaflow cat back exhaust. Don't think high flow downpipe/primary cats are available for the 9th gens yet, but are for the 8th gen.

Once you install just a tune/intake your car will be transformed and feel like it actually has 305 hp.

The OP failed to post which engine his 2014 Impala has - it may have the 4-cylinder engine.

 

[1999 White C5 Coupe]

Hmm, thread title says it all...

Oops - My error. I apologize.