You think AWD will improve GTI or Si? No.
That depends if the rest of the car was properly tuned,car's power output, chassis rigidity, suspension design and dimensioning, torque distribution between axles and is mostly noticed on cars with a power output in excess of 200Bhp. Usually, the more power a car disposes better handling/less scrub in the twists and turns in an AWD car than FWD or RWD (which are generally more fun). You may see a trend of more high-end cars going towards AWD as the years progress.
With AWD you have less wheel spin and torque understeer (a car's tendency to exit the curve by following a trajectory whose radius is longer than the corner's when a car understeers the driver has to steer more than he/she'd normally have to). AWD vehicles have this issue as well, but much less and depending on the driver you can force more neutral driving by "braking late" when entering corners(you do this as well in FWD, but the rear wheels are more subject to sliding and you can't change gears while braking-most efficient when used with "left foot braking"), in order to create a weight transfer, and push the gas pedal while inside the corner in order to allow a "torque transfer" (usually operated by self locking differentials) to the rear wheels. This can be really felt in turbo-charged vehicles. Most drivers with experience do this without even thinking because it "feels good/fun" when driving.
Generally FWD cars are less balanced then AWD and which have better grip and stability. Due to most of the weight being up front, which contributes great inertia forces while in a corner. Since no opposite force is present to spin the car around its vertical axis (i.e. no rear wheel traction) the car has a natural tendency to understeer. Most of the time you won't really feel this unless you push it in the corners - like on a track or really tight exit/entrance ramps.
If you want to see the difference test drive a Mazdaspeed6 and a Mazda 6, Audi A4/A5/Q5, BMW 333/335,etc. with and without AWD - granted these also have engine,chassis, and suspension upgrades as well, but are also heavier cars. Very different cars in the turns, but for commuting to and from work most won't feel the difference.
The advantage of FWD is it is the cheapest and most compact setup with better gas mileage, which is a higher priority for regular road cars - the downside FWD cars have understeer, torque steer and have unbalanced weight distribution. AWD cars are good because they are the most stable in low traction situations, but are also the heaviest, higher priced, have higher mechanical losses (friction) due to the number of gears to drive (differentials,axles,driveshafts,etc.), and have more parts and associated maintenance cost.
I would say the all around best for all year everyday driving is AWD because it balances performance with stability, but it depends on what you want from your car and how you will use it. So you have to make the right choice for you. Just my 2 cents with 3 cents change.
If you were allowed to test drive the AWD at a test track check if your dealer will let you drive a FWD back to back to get a real comparison.
Either way they are both pretty fun!
