If you use a "normal size" tamper you will get pesky grounds forming a ridge around the wall of the basket - requiring a sharp tap to dislodge- and then a retamp..... which will damage the seal against the wall of the basket. But it is evidence of a bigger problem.... the grounds under the tamper will also not be as compressed against the wall of the basket.... and as water takes the easiest route.... channelling!!
The half volume tamp does produce a better seal but must be at a lower weight - just a couple of kilos - or you end up with two pucks sitting on top of each other!
The NSEW tamping method (tamp 4 times at the compass points within the basket) also produces a better seal against the edge of the basket - but each tamp disturbs the puck - so you can get channelling elsewhere.
The best method is one tamp - and one tamp only - using a fitted tamper (like Greg Pullmans. The proof is that pours are longer, the espresso tastes better, and the spent puck is harder to tap out...... all because it is uniform right to the edge..... with almost zero chance of channelling (and you can do the half full tamp if you wish - and that will prolong the pour time even more).