It can easily punch 3-4mm at 25-30mm.
I am now working on a clamping system to hold to material to be punched because there is always some distortion. I am sure these clever guys and girls on here can give you much more information as they already have, however form experience I use a punch made from a piece of bar and a washer or hole cut steel as a die. I have found that the gap between the punch and the die need to be roughly the same as the material you are punching otherwise it's very difficult to punch or you don't get a clean cut.
I have got to be honest with the basic setup i'm using at the moment I wouldn't be happier punching anything thicker than 4mm.
but if your interested i'll keep you posted.