I am not sure these have been done but:

1. Superchips vs Diablo canned tunes- stock vs SC91 vs Diablo 91 (and Diablo 93 vs SC 91)
upgrade cost: $200-$370

2. Exhaust Cat backs for performance (and possibly dB)- including Corsa, Magnaflow, SLP, Borla, etc.
Do these just sound better or are there performance improvements
upgrade cost: $500-$1500

3. The new ported throttle bodies
upgrade cost: $300-$400

The easy bolt-on stuff on our cars tends to be (or claim) about 10 HP- the intakes, the tuners, the exhausts, etc.

One could potentially combine these bolt-ons with the previously researched intakes (or the new Frankentake IV) for a total cost of around $1200-$2500 for a performance improvement of ????? (well that is a good question).


Clearly this is climbing into a head/cam package ($1500+ installation of $600-$1200) but the "bolt-on mods" probably don't hit the warranty if that matters to you.