Tune w/ HFC's
 

I got my FI HFC's installed and I've seen dynos of people gaining 20hp with a full cat-back and tune, so does anyone know how much of a gain I could expect with just the stock exhaust, and the HFC's? I haven't found before / after dynos of just HFC installs. I'm trying to decide if it's worth getting an upRev tune since I don't plan on doing any more NA power mods.

I'm guessing my money would be better spent on a set of R-compounds and lightweight wheels as far as Autocrossing goes, but I also want to take advantage of the HFC investment the best I can...

Why install HFCs and keep the stock exhaust?
That Y-Pipe is restrictive. Get a nice cat-back system to go with your new cats... otherwise you wont be gaining much more than a sound.
Those 20 HP gains are with a cat back exhaist I believe.

I think you should see a 5 to 8 hp gain with the HFC's alone. In my opinion the stock cats are the ost restrictive part of the exhauset system. If you have a high rev motor (05 and up) then the best thing for you to do is get an Mrev2 + plenum spacer. If you dont have a high rev motor than just get the spacer. I dont know if you alredy have a z tube and a K&N drop in filter but that would be the first thing I do because it is only about 120 bucks.

Everything I read said that the cats are the most restrictive part of the exhaust, so I anticipated that I'd get most of the gains, minus a little bit that the cat-back would give me. If I get 10 - 15 out of 20 hp from the cats, then I can't justify spending another grand for the cat-back. Plus, the G is my DD so I'm not willing to have a constant drone in the cabin from a full cat-back.

I've got an 07 awd sedan so I didn't think any of those fit the v36 sedans. Same with the z tube, although I guess I could get a couple K&N filters if it really matters.

Get the Mrev2 and plenum spacer. You might see 20-25hp gains from that alone. It's the best bang for your buck. Also get a K&N drop in filter. Its only like 45 bucks

I didn't think the Mrev2 and plenum spacer were applicable to the HR engine (which is what we have) - am I wrong?

The Mrev2 is applicable specifically for the High Rev engine. It does not work as well with 03 and 04 models. You get the most gains out of it if you Do have the HR engine. That why people that have 03-04 models only get the plenum spacer without the Mrev2.

Motordyne says it's for the 287hp rev-up engine which is the VQ35DE - not the 306/311hp VQ35HR engine that is in the '07 G35X.

I tried to order it from them and they told me it's not for the VQ35HR ('07+ Sedan)

OP, Uprev tune helps the car gain low~midrange power. HFC's will help open up the car at high RPM, tune will help the car in low RPM (peak power will also increase as well). HFC/test pipes and Uprev tune are the best power makers you can get if you're on a budget.

HR is different than Rev-Up. you're confusing the DE Rev-Up version with the HR, which is really a totally different engine. Plenum spacers and Z-tubes don't fit the HR.

Q8y, have you heard of people doing tunes w/ just HFC's or cats w/ an intake and getting decent gains? I'd like to get a tune for reasons other than the power like the throttle response improvements and WOT at any speed too, but if I'm not going to gain 10+ HP from the tune then it's probably not worth it to me.

Honestly I would wait to get a tune till you added a cpl more bolt on parts. Its expensive to keep getting a tune whenever you add something new.

Indeed :)

Although you seem to doubt the cost effectiveness of getting a tune, I would definitely get it done because it's more about having your engine run as best it can rather than just squeezing those extra horses out of it. The Uprev tune is a must in my opinion, plus you get several free updates from them when you add more mods and send them your Cipher logs.

I don't anticipate getting any other bolt-ons because of the cost/gain ratio being so minimal, and because I don't plan on keeping the car for more than another couple of years. Is there anything else worth buying that'll net more than 1 theoretical hp that's a bolt-on?

I'm all about having the car run the best it can, but if the tune isn't going to make a change that I'll notice, or increase the "fun to drive" factor, then I'll pass on it.