AFM is overrated in these GenIV motors, IMO. The fuel savings is minimal and good luck trying to get an exhaust setup that sounds decent when AFM is active. These things drone like a **** if your exhaust isn't tuned properly. I know a lot of G8 owners that have eliminated AFM and not seen any change in fuel mileage. In many cases mileage improves with the addition of a CAI and a tune. I would suggest going with the GENIII for your swap. That is what I plan on doing down the road.
yeah, fuel mileage has more to do with driving style and a proper setup than gadgets. although as a science geek, I like the gadgets and AFM is a pretty interesting setup.
for example, bone stock (stock L31, stock 4L60e, open carrier 3.73, stock 29" tires) my truck would get and average of ~16mpg peaking around 18mpg, thats 100% bone stock...
added shorty headers, a CAI, truetrac posi, ZZ4 cam, custom external MPFI manifold sporting 24# injectors, 411PCM, and starting doing some tuning on 'er, now averaging ~15mpg peaking around 17mpg...
later swapped to a T56 6 speed trans (double overdrive FYI), gearing still at the stock 3.73, removed the CAI and put the stock air box back in, more tuning, now hanging out at ~12mpg peaking aroung 15mpg...
Changed heads, added supercharger at 6psi, changed injectors to 40#ers, pulled shorty headers and replaced with full length headers, full 3" true dual exhaust, removed distributor for 24x setup with LS2 coils, more tuning, steady at ~11mpg peaking at 13mpg...
regeared to 4.88 (still with the same trutrac still with 29" tires), more tuning...back up to ~15-16mpg, peaking in the high 17mpg's...
this is all over the course of 3-4 years on my daily driven truck, with the MPG's calculated at every fillup and maintained in my mpg spreadsheet. most fillups were done at the same station, and using the same pump as well.
IMO my results show that a proper setup and driving style have WAY more to do than mods when it comes to fuel efficiency. after all, I went from bone stock, to a highly modified supercharged setup, and didn't even lose 1 full mpg in my daily driving. sure there was a time in there where I lost a decent amount and only averaged around 11-12mpg, but my gearing setup wasn't correct for my trans, and once that was corrected, the fuel efficiency came back up.
I doubt AFM would affect it that much if I were able to add it, MAYBE 1mpg on the highway only. but its not like we'll ever know on my setup at least since there is no real feasable way to add AFM to a SBC.
if someone wanted to do a motor swap with AFM because the setup is interesting and science is fun, then I'd say go for it, learn and have fun. but if someone was wanting AFM simply to gain MPG's I'd say look elsewhere for something more worth your money.