Paolo,


Increasing the tuned length of any tuned pipe system will decrease the RPM, at which the maximum boost will take place.

If you are using a relatively large propeller, spinning around 8,000 RPM, there will be little difference, if any, in maximum RPM and it may even take the engine a fraction of a second less, to get there.

If you are maxed out, with an 18x6 prop at 9,000 RPM, however, the top end performance may be reduced a bit, by this slightly greater tuned length.