how much of a drivetrain loss do us FWD owners have?? (preferably manual trans.) wouldnt it be less than 15% ??

That's a pretty vague question to ask. There's always going to be power output variations between techniques and technologies used by different manufacturers. You can have an MF (front mid-engine) layout, FFL (front-engine longitudinally mounted) layout, or FFT (front-engine tranversely mounted) layout. The most commonly used one today is the FFT, with a 4-cylinder inline in most compact cars.

If your question is whether FWD is the most efficient, then yes. It offers the shortest distance and most direct power output into the CV joints. RWD suffers a little more output loss because of a long driveshaft and universal joint. AWD suffers the most because of the added weight and complexity of the powertrain system.

Manual is ideal because you're directly connected to the gears. Traditional automatics suffer from a slight power loss because of torque converter. Dual-clutch transmissions theoretically offer benefits from both sides (no torque converter to sap power and fastest shifting response).

If you want exact numbers you're going to have to do a dyno run. Compare the crank horsepower/torque figure published by your manufacturer with the wheel horsepower/torque figures from a dyno run. There are probably websites out there that publish stock dyno runs for your specific car if you don't feel like paying for that.

You couldn't get a better answer then what mmmose posted, bit if you are about %
GENERALLY
Fwd losses about 10-15%
Rwd 20-25%
AWD 25-35%
These aren't MY numbers when I used to do research those where the numbers I generally saw posted

Just to give you an idea, sti's do put out305at crank but if you go see their stock dynos they are as low as 215-225 AWHP on the other hand the mazdaspeed3 turbocharged FWD same engine as he 2005-ll2007 focus ST, has a hp of 264, but when you dyno it stoc has about 215-225whp as well. That just goes to show a lot.

Edit: When I did research on this topic, I came across this website.

http://www.mk5cortinaestate.co.uk/calculator4.php

NOTE TO SELF, this will just get rough number and estimates not the perfect thing, I did some numbers and they did seem to be around the same numbers, so it is respectable, but untill you get your car ad throw it on a dyno this is a nice little thing to see what your car is roughly putting down.

I know the % figures are tossed around quite a lot, but they really don't make much sense.

Your drive train should not really be sucking a variable amount of power. It is going to vary for not only every single engine and model combination out on the road but also per car. A car with an old worn out drivetrain may waste a little more energy then a new car of the same exact model. So those percentages should be taken with a grain of salt and a rough rule of thumb.

For instance, if I make fairly "minor" mods which do not affect parasitic losses on the engine then the % power loss in the drivetrain should not change. (i.e. no belt driven supercharger).

For example, say a high-flow cat, a K&N panel filter and Tom's tune can get me 15 extra hp on my SVTF (don't focus on the number, its just an example not a factual claim). Then by the 10-15% FWD powertrain loss quoted above would imply that my transmission is taking 1-2 of those 15 horsepower I gained by mods.

But my transmission and clutch are EXACTLY the same as before, no changes. So why would the transmission steal an extra 1 or 2 horsepower from those mods if nothing changed? Logically it should be sapping the same amount of power it did before.

From a different perspective, those percentage figures don't really account for RPM either. If my car makes 100 hp @ XXXX rpm, then why would the transmission and drivetrain sap more power from the car when it makes 150 hp @ YYYY rpms?