I've had a reasonably quick/cheap V6 before, so here's my advice:
Make it lighter, make it more aerodynamic, and make it lighter.
That's where all your CHEAP gain is going to be, without opening the motor or going down the expensive (relatively) path of FI.
Here's what you can do to make it lighter:
manual conversion (50kg) (+10kw at the wheels)
sound deadening and rubber matting (20-30kg)
aircon (20kg)
power steering (5-10kg)
towbar setup (15kg)
spare tyre and jack (15kg)
little things here and there (10kg)
Then make it more aerodynamic
lower the front a little more than the back ($150)
front underbody diffuser (DIY)
small air diffusers in front of the wheels (DIY)
And you'll be shocked how much more fuel efficient it is, how much more stable it is at high speed, and how much quicker it picks up off the line.
Then get some BMW alloy stockies for less rotating mass. Get good sticky tyres, and then throw some 3.73 or 3.89 diff gears up the arse with an LSD, and you'll be in business
Cheap, fast, and VERY enjoyable to drive.