I would usually charge $300.00 plus ship/handling to Groove an Edelbrock 4 BBL, or a Holley. But I always recommend to anyone savvy about engines and cars to get the PL kit and learn it themselves.
By itself the Groove has shown substantial improvements to carb's. But anything Carbureted can definitely take advantage of other improvements and mods. Edelbrock Performer 4 BBL's are easy to tune, jets/rods are available for 'em, they are my favorite 4 BBL carb. FAR more forgiving than a Holley, IMHO. Performer/AFB carb's tolerate dirt, heat/cold, in other words-the Real World- FAR better than a Holley. They are much easier to tune and service than a Holley. Now, there is a newer design Holley, that is sort of like a Ford/Motorcraft carb, in that the top comes off to expose the fuel bowls instead of the old style Holley w/bowls and metering blocks at each end. No one has ever posted here about Grooving this newer Holley, I have never worked w/ one. The one advantage Holley old style carbs have over the AFB/Performer is the Choke. Holley's have the best Choke designs around in my experience. For smaller engines, stock type one and two barrel Carb's are a Crapshoot, many of these are un-suitable to Groove, and tuning parts are often non-existent. BUT- Weber carbs, at least the DGV, DGEV, DGAV ones ARE Groove-able, I have a Grooved DGEV on my 85 Subaru (Subie). Subie is my Avatar pic. Converting to a Weber is it's own topic, there are many different Websites to address this. Details....
In response to your question on potential improvements to MPG- well, it depends...
Last summer, one night in perfect conditions, I achieved over 70 MPG w/ my Subaru, that is equipped w/ a Grooved Weber DGEV 32/36, and several other mods. I don't get anywhere near that in day to day driving of course. In the ReaL World, it's difficult to get over 10% increase using standard techniques, excluding the Groove. I claim that my Subie is the only one like it around- and I stand behind that claim. NO-Body has done the combo of stuff I have to a Subaru like mine. Not to be egotistical, it's simply that I've done things most never would. (Plasma Ignition...)
Look in the Index for Heysoundude's posts re: Pre-Groove Vehicle Prep for some solid ideas. IT's about killing gremlins, and reducing parasitic losses too. Just eliminating manifold vacuum leaks can do wonders. To get real good gains it's often about how much effort research and $$ you can invest. Gremlins are like insects, they're everywhere in cars!
I'd say get rid of problems and try to make the vehicle a solid unit, then go for the trick stuff.
Tracy G