I have never heard of that. We do have a Mover, but it was fitted by our dealer. However my husband has fitted various extras including additional sockets and shock absorbers, and our dealer has not commented on this.
If anything he did caused a problem with something in the van then that could be different, but that is all I can think of.
Fitting a Mover does not involve drilling the axle, the only possibly intrusive bit is the electrics, and even that is not that complex.
Who told you this? And what do they mean by "changing the spec"?
It will add weight to the van, that has to come from your payload, but you could probably get an uprated chassis plate to increase the MTPLM.