which holes - you mean the grouphead water holes beneath the screen are towards edge of puck ?,I moved to just 14g dose (rather than just filled to the brim with ground), proper size basket and realised that the 4 holes on the gaggia is on the outside. Meaning a curved tamper would have less pressure on the outside.
I had a curved tamper, my basket is, now, convex too, maybe matching the profile, and, keeping coffee depth uniform.
actually seems an interesting mathematical problem where to locate those holes to give the jets uniform coverage of the puck area