i don't have experience with the fi's but i had a p3 in my jetta, and it was really bad....
the 15's might be better, but i had a 10 with 400w rms on it, and i started saving for something else the day i put it in
i put it in with the mentality that "i don't need TONS of bass, i just want a nice added kick" but it didn't even fill that purpose, honestly
it may as well have not been there