Interesting thread. I found myself in the same boat as OP. Had a 2005 G35x prior to my G37x. In my last G I installed air intake, plenum and throttle body spacers, HFC's, and exhaust, and while I loved the sound and the gain in mid range power, I absolutely despised the lag I felt when launching from a complete stop. This surely isn't the best comparison though, considering that I came from a car w/ single air intake/throttle body to the newer dual intake/TB design.
Nonetheless, on my new G I've purposefully avoided any mods that could potentially affect low end torque for this reason. W/o a tune, my butt-dyno will barely notice the difference in mid/upper range power, but will absolutely notice the loss of low end torque off the line.

One thing to keep in mind is a lot of the people giving input here have RWD Sport G's w/ manual transmissions. Us AWD A/T guys launch off the line differently than they do. I don't have any information or details on this, or know who has the advantage, but definitely something to keep in mind.

I personally went w/ a muffler delete and kept the stock size piping and am content as is. Humble amount of noise at lower speeds, and essentially dead silent at highway/interstate speeds.

 

Stillen has documented HP/Torque gains over stock: G37 Sedan Gains 56 HP After Visiting STILLEN Performance Shop

56hp on this sedan with a CBE, intake and tune. Not too bad.

I'm guessing the cost for all of this would be in the 2K range.

 

Maybe.

Gen 3 - $560
CBE- $1200
Tune License - $300 ~
Dyno Time - $300 - $600 ~
Labor - $700 ~

Say starting around $2800...about.

Sean

 

I've got a full FI exhaust on my G. For over a year, it was just the RHFC's attached to the OEM cat-back, and there was a butt-dyno improvement everywhere, low-med-high.

This Spring I had a 2.5" FI catback installed, and was very surprised to feel a bump in mid-range. Low & high end felt the same, but mid-range was pulling stronger. It just was.

I'm still untuned. And honestly, I think every car is different. But that was my experience.

 

Thanks.

I was assuming that Stillen themselves would discount the labor and dyno time since the full package was being purchased. Of course for them to do all of the work, you would need to live close to their facility here in Socal.

 

That's right, I seem to recall that you have the 2.5 inch FI setup with those gorgeous mufflers. I tried to find those as a stand-alone item and failed.

 

I assume that means you reached out to Fast Intentions, and they said "no" in response to a muffler-only purchase.

Have you contacted Sebring Tuning directly, to see if there is anyone who retails their mufflers as stand-alone products?

Sebring Tuning Gallery

I suspect they're supplies only, but I don't really know. Sure is pretty hardware, however you get it.

 

John - Thanks for the Sebring link. Very informative. Not sure why I didn't go there until now for more research into the FI setup.

Sean