As Stu has commented, any speed density ecu cannot correct for a change in air flow for a given air pressure. This means that if you change your plenum on a cossie engine you WILL need a remap.

In laymans terms this is because the speed density system uses estimated air flow (Or V.E. to be precise) based on rpm and air density. If you increase the engine air flow (i.e. +% on V.E) for the same plenum pressure (e.g. by spacing the plenum away from the trumpets) then the engine will run leaner and hence need remapping.

In addition increasing plenum volume has a dramatic effect on transient fueling as the volume of air between the cylinder and throttle body has increased. On L1/6 this can be difficult to do anything about and can result in slight hesitation between gear changes, but L8/P8 and better can all be remapped to cope perfectly with increased plenum volumes.