yeah a 4" down pipe will eliminate some backpressure and maybe some low end torque. i have the 4" turbo back though and I dunno.. it's still very torquey. especially with the chip.. i can sit down low in the 1200 RPM range coasting no problem at like 40-45mph.
the intake.. tough to say. it might help a little but without a tuner intake and exhaust aren't going to do as much. a tuner will really take advantage of the increased flow. you should notice less lag with the intake.