First of all, this system was never fitted to the 155. It was introduced on the 156 and 145/6 around '97, and only the 1.8 and 2.0 16V engines have this system, not the 1.6 or the V6. The engines have motronic M1.5.5, instead of the M2.10.3/4 fitted to the 155. So it's not possible to retrofit it to a 155. Well, anyhting is possible given a bargeload of dosh, but there you are.
As for the "variable" bit, it's actually an on/off system. Below 4960 (really!) rpm, the air has to travel through a curved inlet pipe from the plenum to each inlet port. The length of this pipe is such that the pressure waves are more or less in phase with the valve opening intervals. This gives good torque.
Above 4960rpm, a flap is opened which allows the air to take a shortcut, so to speak, bypassing the long curved pipe. This maximizes the amount of air available at the inlet port, for maximum power. At these engine speeds the pressure pulses are too slow, and actually reduce cilinder filling because the pulses are out of phase.
So it's either long or short inlet pipes. The lengths of the two inlet tracts are optimized for their respective rev bands. This means that the length is a compromise across the rev band rather than the exact optimum for a given number of revs.
Ideally the length should be varied continuously across the revs, but that is (was?) only done on racing engines. These were fitted with sliding intake trumpets operated by the accelerator.
As for the tuning possibilties, unless you change things like the camshaft timing, valve size and lift, etc, I wouldn't think that changing the 4960rpm to something else will help (much). I assume that the factory engineers figured out where the ideal cut-off point is. Then again, it may be a compromise between engine power, emissions, driveability, mpg etcetera. Which leads to the chip tuning issue, which I won't go into here.