I have been in car audio on and off for the last 5 years. My biggest suggestion to you would be to see/hear what you want before you buy it. Under the seat subs, in my opinion, are very expensive to install and can be very difficult as well. Cars that have powered seats mount the motors right where you are talking of putting the sub. Also, if the seats aren’t powered, there is still VERY little room under there. In order to have a subwoofer function properly in a box it needs to have a certain amount of area in the box specified by the manufacturer. The space under a seat doesn’t allow for this in most circumstances.
My suggestion on a sub would be to either mount it in your trunk in a box enclosure or look into a stealth box which is much smaller and takes up less room. Not to mention they look really clean.
Also, street glow (under lighting) is a bad idea in NY (or anywhere for that matter) because of the weather that we see and it’s a waste of money because they break relatively easy with out adding function to the car, just my opinion.
Spoilers are the same way, just a waste of money and they don’t do much. The vehicle was engineered to be aerodynamically sound and putting an aftermarket spoiler on your car increases your drag effectively making your car slower. They are pure form with no function.
As long as what you are putting into the car can be taken out with no adverse affects, it should not void your warrantee. I would still make sure to check with your dealer to find out what is allowed.
As far as finding an amp on Ebay, I would not suggest this. While you may get lucky and get something that is worth the price, most of the time I have found that audio products on Ebay do not stand up to what they are advertised as. Best Buy has a good warrantee plan. For $40 more than the purchase price of the product, you get a 4 year warrantee that if the product stops working for any reason other then physical damage; you get the purchase price back towards another product. This happened to me and I was able to buy a brand new amp 3 years after my first one for about $20 more that was a better quality than the original. Crutchfield is a good company to purchase from because of there large selection and have been in business for a long time. Although as far as I know, they do not have the warrantee plan that Best Buy does.
If I recall correctly, your car is a 2.5L NA motor and the WRX is a 2.0L Turbo. When you get into upgrading an NA to a turbo, there are a lot more things that can go wrong. I would strongly suggest not putting in a turbo as it is a lease car and more then likely something will break that will void the warrantee.
I am a firm believer on function over form. So if it increases the driving experience, go for it.
Good luck
Jake