I apologize, however your wording is somewhat vague right around the actual question itself. However my interpretation says that your question was: "What level of advancement would today's aircraft be at, had modern piston engine technology existed at the time WWII aircraft were designed."
My answer is, not too different. Indeed we have TSIO-550-C flat-six engines with GAMI injectors and electronic ignition, but what does that give us? Lets take a look at arguably the greatest airplane ever built, the Douglas DC-3; she first flew over 76 years ago, 400 still remain in service worldwide, and no other airplane has come to match what it does. This is a plane powered by two Pratt & Whitney R-1830 radial engines (first run in 1932, 80 years ago), it was designed without computers, calculators, just hand-drawn math. It can deliver cargo cheaper than any other plane, flies as fast as any light twin (Beech Baron or Cessna 310), and can carr 25 people. It can land on snow, water, gravel, tarmac, dirt, grass, what have you. Modern engine technology wouldn't have done any good for the DC-3; the Bastler BT-67 turboprops have been made, but the radials are still better. (apologies, its a BT-67, not -17; edited to fix)
Let's look at the de Havilland Beaver, powered by a Pratt & Whitney R-985 (first run in 1929, 83 years ago), it was first flown on August 16, 1947. The Beaver can land shorter than most any production plane, can haul six people, or 2,100 lbs, land on snow, water, or anything in the bush. Although being in service long enough to be on medicare, no plane has matched the Beaver, and the old radial engine is still the best engine for the beaver, although the PT-6 sees limited application. Nothing today would have made the Beaver any better, if it could have, somebody would be out to make a fortune.
Modern technology indeed helps make a single go fast with its gear down, and the autopilot is one of the greatest tools pilots have, but is it really better? We put new avionics in the old airplanes, and voila! We have the best thing money can buy. Our technology makes big airliners more effecient, and for new planes being built for city-slicker general aviation, sure, its great. But when it boils down to being better, a Cessna Corvalis TT doesn't compete with a 50 year-old Baron, except for the highest price.