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You guys may be a little inundated by all of the "great" info out there on the web, lol, at least on what makes power and how much. I'll do my best to give you our experience on working on these engines for the past few years.
For the automatic 6.1L, a CAI will give you about 8 to 10 rwhp at the most. A cat-back about 10 to 12. The tuners are actually doing much better, hitting 15 rwhp and sometimes more. Custom tuning usually doubles those gains. Stock you're at 360 rwhp on average. A catback and a CAI will have you in the mid 370's.The tuner will bring you up to 390 and a custom tune can bring you usually over 400 - assuming that you're running in good air with low humidity and in the low 70's.
The 90mm throttle body's don't make huge gains on a 6.1L, plus you will need to have your manifold ported to 90mm, too and decked to use it effectively and the peak numbers would probably only be around 5 horsepower at peak but you risk losing low end power unless you pay a good bit of money for custom tuning where the tuner spends a lot of time tuning down low. I only use the 90mm t/b on the strokers we build since they are cost effective and as hard to tune out the driveability effects a 90 causes on a stock motor (tuning does eliminate most of this). To effectively use the 90mm on a stock 6.1L, expect to pay about $2,000 to get 5 (maybe 10) horsepower.
Fuel mileage: increases will be seen from any exhaust work you do. Cat-back, headers, high flow cats, they all add to your mileage and you may see 3 or 4 mpg better once you're replace the entire exhaust. CAI's help a little, too. The more efficient your engine runs, the better your fuel mileage. Our 426 stroker is capable of 27mpg highway on the heavier, less aerodynamic LX cars because the engine runs so much more efficient.
Tuning helps the mpg's, too. I've fixed tunes for customers where the highway parameters we're way off. The last one I did about 2 weeks ago, the customer couldn't get over 12 mpg highway. A quick change and he was well on his way back up climbing fast towards 20+ mpg on the highway. These cars can be tuned for the highway just as easily as they can be tuned for WOT. Personally, I think the biggest problem we face on the mpg is the fuels most states' use. Ethanol kills mileage.
Just my $0.02
Joshua
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