The simple 3 pipe modification to the air intake trunking is normally carried out in conjunction with a foam replacement air filter element. The greater the pressure of the air going into your engine equates to a few extra horsepower, also the cooler the air the better as it has a greater density.
Although an induction kit allows more air to be drawn into the engine, it also draws in a lot of hot air from inside the engine bay. There are induction kits on the market with additional tubing thus positioning the air filter nearer the front of the car in order to draw in more cold air but it is important that the car does not draw in any water as this would seize the engine.
By removing the ends of the three pipes, you are simply creating a greater area for the suction of air into the engine, this can only help. If you remove the end of the pipe in front of the radiator first, get someone to rev the engine while you feel the additional air being sucked in.
It should be a simply task to carry out, I understand there are quite substantial instructions for this on vectra-sport. An ideal set up would be to have an extended induction kit with the filter positioned where the standard airbox normally is, this could have some tubing facing it from the front bumper or an intake vent on the bonnet.
By shortening the distance the air travels in, you also shorten the distance for the noise to travel back out from the engine. The three pipes are there for noise suppression so do expect a little more of a roar from the car under acceleration.
Maybe this will clear up some of the issues that are arising on this board
I have just carried out this mod. It is quite an easy task. For those who may wish to try this here is a step by step guide, (at your own risk)
Pipe 1 is behind numberplate. Remove 3 m8 bolts from center grill between headlights and it un clips. You will be able to see the pipe. unclip and cut the end off. Re-assemble.
Pipe 2 is the "normal" path for air intake and is just above pipe 1 by the bonnet catch.
Pipe 3 is between drivers side bumper and wheel arch liner. Remove indicator plug and lamp, and unclip the indicator from the headlight. Its a whiteish clip. Indicator will then push straight out. Now you will see a black pipe which is blanked. remove the Tx25 screw and unclip. cut the end off. Re-assemble.
I also shortened the pipe in the bottom of the air filter housing.
I kept the off-cuts incase i wished to blank them off again, but when you take them off you will see they have all the part no's on anyway, so new pipes can be bought easily.
The car now has 3 paths to draw incomming air instead of 1.
First impressions with these mods, no noticable noise increase, on a short test run, allowing the car to reach normal running temperature, it does appear ro run smoother.