Here is a list of tested mods originally posted by 4DRHTRD in the "power per mod" thread and he updated it in October 2013 it discusses exhaust mods and ranks the mods by bang for the buck. the link and a copy of the post are below:
http://www.ecoboostperformanceforum.com/index.php?topic=808.304DR's post is below:
Thought this might be a good topic for new owners to review:
I'm going to rank these by my opinion of best mod by power per dollar spent.1. Tuning - you can pick up 30-70HP here pretty easily depending on what fuel is available to you and if you want to run race gas or not. Figure around $500 with tuner. The lower octane you can run the lower HP you an pick up thus the range. If you really want to maximize your power with tuning doing a dyno tuning session with your tuner will pick up another 10 to 30HP (IMHO) but will cost you a few hundred dollars more. If you're looking for ever HP safely this is the best case scenario.
2. Meth injection - you can pick up another 30-60 HP here depending on how aggressive you want to get with timing and your tuner. Cost between $200 and $1000 depending on how fancy you want to get with controllers, extra bottles for storage etc.
3. Turbo upgrade - If you have a tune and meth injection (my suggestion) or an aux fueling solution (discussed later) you can upgrade your turbos. Worth another 30-? (most HP now is 600WHP with aux fueling)HP. Currently the upgrades upgrade either your compressor wheel and/or add an aftermarket wastegate. There is also a full turbo upgrade coming this fall or winter so no one knows the true maximum output of the turbo upgrade scenario yet and how much power the block will take. Currently $1500 to $2000 for the two stock turbo upgrades, and $2200 for a true turbo center section upgrade.
4. Downpipes - luckily multiple companies offer these now and they're getting cheaper, between $700ish and $1000 for around 20ish HP and quicker spooling of the turbos.
5. Exhaust - $100ish (remove 3rd cat, resonator and mufflers with straight pipe) to $1000+ for full stainless catback exhausts. Another 20ish HP here.
6. Intake - jury is out on this one, we've seen these drop ET/MPH from doing intakes but hopefully we'll be coming out with a new intake tube per turbo with a cold air or sealed stock style box soon. $200ish to $700
Optionals not normally used (my crazy mad scientist mods)Nitrous - I've personally ran a 75 shot on my Flex and picked up .6 in the 1/4 mile. You can pick up a used kit and controller on Craigslist pretty cheaply and install. For all brand new gear it can get expensive. $250 (used parts) to $$$$$ for new.
Auxiliary fuel system - I'm the only one who's built one so far, to replicate with what I know now around $2800. Gives the ability for unlimited fuel without the HPFP sagging under load. Needed in the long term for the true turbo upgrade or if you want to run E85. This includes injectors, dual in tank fuel pumps and a modified intake manifold or intake spacer (working on this one)
E85 or E85 mix - Auxiliary fuel system required for this one as the stock system can't handle ANY E85 mix without running out of fuel. E85 let's you increase the timing massively and run "hotter" boost while cooling the intake track off as well. An alternative to running alky/water injection but a true fuel. Requires dual in tank fuel pumps to replace factory pump. This fuel has allowed me to get to 600WHP on just a turbo upgrade and downpipes/cutout.
Exhaust Manifolds - Now that I've hit 600WHP, the small size of the factory exhaust manifolds is starting to get in the way, need to find a way to replace stockers, this might open up a way to also install a T3 turbo swap.
Water to Air Intercooler - 600WHP has caused me to start looking hard at the factory intercooler and replacing it. An aftermarket water to air will be the best scenario as there's just no good way to fit a big air to air in the front of the car. I'm guessing at the current 26PSI I'm at that I can pick up 30-50WHP here.
Dual-in-tank fuel pumps - Auxiliary fuel system required for this one as the stock system can't handle ANY E85 mix without running out of fuel. E85 let's you increase the timing massively and run "hotter" boost while cooling the intake track off as well. An alternative to running alky/water injection but a true fuel.
