I ask one of the Subaru dealer in town today. They quote me it's gonna cost me $1240 for 6 years/ 80,000 miles extened warranty (Gold Plus Plan) from Subaru. Is it worthy it? I know Subaru don't have "powertrain warranty" like other Japanese car. (I owned a 2000 celica gt-s before)

I have a 05 WRX Wagon, 24k miles. I'm planning to do a full supsension mod and silencer delete only. Probably only mod I will do on my car.

 

Subaru does indeed have a powertrain warranty, 5 years/60K miles. So you are only buying an additional 1 year/30K on the powertrain, plus extending the warranty on other parts of the car from 3/36 to 6/80.
Looking at it that way it's not such a great deal. You need to look at a few things- how fast you expect to accumulate mileage, how much more you plan to mod the car (your suspension mods will probably render any suspension-related warranty claims void), and what the warranty will actually cover (read it over very carefully).
You can also do some shopping around for better prices on the Subaru warranties and ask your local dealer to match the best price you find. Some will do that.
And don't rush into this, you have until the end of your bumper to bumper warranty period (3/36) to purchase an extended warranty.
In general though, these warranties are a good deal for the dealer who gets to keep your money, and not such a good deal for you.