Blackflag, 4 valves per cylinder does not automatically equal better fuel mileage. GM's most fuel efficient V8 engines are the pushrod line instead of their overhead cam models (and I'm not taking into account the AFM versions). I'll give you a few real life examples. GM took their 60 degree V6 engine and added a 4 valve DOHC head to their engine and increased the displacement to 3.4 liters and the fuel mileage dropped from 29 mpg with the 3.1 liter to 26 mpg. Granted the displacement grew and they gave it a slightly shorter 3.43 over the standard 3.33 in the models it was equipped with but later a OHV version of this engine line was offered in the 2000-2005 Impala and it was rated at 32 mpg with gearing in the 3.07-3.05 range.

When they offered the "Shortstar" (V6 Northstar 4 valve DOHC) as an "upgrade" from the 3800 series II in the Intrigue, the fuel mileage dropped from 30mpg to around 27-28 while offering similar gearing; meanwhile, the supercharged Grand Prix GTP offered shorter gearing with their supercharged 3800 pushrod and got equal to 1 mpg better on the highway.

Most recently, the 3.5L "High Value" (2 valve OHV) powered Pontiac G6 only featured a 4-speed automatic and got between 30-32 miles per gallon, then they offered the 3.6L "High Feature" (4 valve DOHC) version in the GTP and now the GXP with a six speed and despite similar gearing the best the combo could muster is 28 mpg.

The same thing is true with Chrysler, the best fuel mileage the 3.5L (4 valve SOHC) received in the LH cars was 26-27; meanwhile, the 3.3L normally got 28 (2 valve OHV).

Therefore, there are no guarantees that it will improve the mileage. I would tend to believe that it would do the opposite.