i think your situation is fairly common... there are many people lusting after the S4 who own the WRX
. The WRX is very easy and cheap to mod... you can get a manual boost controller (mbc) for $5 to $50 and you will be able to run low to mid 5s 0-60 and high 13s in the 1/4. Then there are various good engine mods out there that go for $70 for an uppipe (I think that's right???) to a couple hundred for a turbo-back exhaust. They won't help the straight-line fast as much as the mbc though. Then you can get new bigger turbos that will spool faster and make you go faster for less than $1k, there are also various manufacturers with stage 0, stage 1, and stage 2 kits for a couple thousand dollars (I don't know what they do but I know they're out there
). Of course all the supporting engine mods are also available, like bigger fuel injectors and lighter gas tanks.
Then, there's a ton of different full suspensions available but they are relatively pricey, like most full coilovers will go for $1k - $3k (I think). If you don't want to do that, you can easily just upgrade the anti-roll bar for $100 - $200. Wheels are sorta hard to find because Subaru uses a high positive offset (+48 to +53) but there are a variety of competition-grade wheels available if that's what you want. There are a variety of brake kits available for a variety of prices, but you'll have to upgrade to 17" wheels if you want 4 pot calipers.
The weakest part of the whole system is reputed to be the transmission, which supposedly runs into "reliablity problems" when you start pushing 280 hp, whatever that means. But you can easily go from the stock 227 hp to 300 hp or more for just a few thousand dollars. And then you'll have an S4 killer
. Basically the WRX is very easily and relatively cheaply moddable in just about every possible performance aspect.
Although, if you are looking for a killer mod car, the 2.5RS would also be a good choice. 6k less than a WRX, and you can throw a turbo on it for a 2.5L turbo engine that will, when tuned properly, beat a WRX. You can easily replace the brake rotors with the WRX rotors, and maybe the rear differential with a WRX LSD, and you'll be all set.
In the end, mod dollar for mod dollar the S4 will be faster because it has a bigger engine and more turbos, but total dollar for total dollar the WRX would be faster. Though, total dollar for total dollar a motorcycle will be fastest
.
What about a used S4? Personally I wouldn't buy a used sports car, but it might be the cheap way to the car you're looking for. You could probably get a 2000 S4 for low $30k.