While still a realtive newbie at the whole coffee making process I am drawing closer to the elusive 25 second shot. My view is that the tamper is v. important. I could get reasonable tasting drops using the default plastic tampers that came with my previous and current machine but once I got a proper tamper it made a big difference.
What I found is critical is the first tamp which helps ensure an even puck- I couldnt achieve that with the plastic tamper. I now tamp lightly and check that the top of the tamper is level with the top of the basket then do my second tamp. I now regret not having spent the extra money on a Pullman tamper as they have rings on the edge to help ensure the tamper is level. Plus you can buy a matching basket which ensures the correct fit. My current tamper is a bit smaller than the basket so some grounds remain clinging to the edge.