Geldap is right in saying there will be a variance in swingweight when butt trimming a driver.
However, we are talking about a weight that is relatively insignificant and would not be noticed by most people even if 2" was taken off. To suggest that 1/2" removed from the butt would affect performance at all, is a load of balderdash - most of us don't grip with such precision each time we play a shot and would be within 1/2" short or long of the alleged optimum anyway.
What concerns me more is the suggestion that we should re-weight the head in some way. This was OK in the days when drivers were solid wood, or aluminium.
Now most modern heads are very carefully designed, within allowed tolerances. The L4V that I use pushes the boundaries on Moment Of Inertia, Centre Of Gravity, Coefficient Of Restitution and overall size . . . any tampering with the head itself, with gobs of lead tape, would potentially render it illegal, as it would on many other equivalent heads.
Also, if the weight is placed in areas where it was not designed to be, it will affect the performance - far more than the percieved weight of a fag packet in its apparent heft (swingweight).
. . . and swingweighting is just a way of matching a set of clubs to the preferred heft of a user, be it a poof or a gorilla. On its own it does not affect performance - swingspeed does.
There is no defined correct swingweight for any given club, be it a driver or a sand iron - and certainly not for a putter. There is a collective agreement in manufacture that historically assumes males will generally prefer around D2.