HI id be tempted to clean the transfer port up with some wet n dry and smooth out best possible [ air flow better wen it not disturbed] I also radiuse the comp tube side and some tunners put a very very slight radiuse on the breach side as well. your right about the transfer port size .i25 [3.2mm] so your not far off .try to get a new type hw piston seal as thay are said to be very good [you might have to size] not to sure wat you mean be a leaky comp tube as hw80 don't suffer from this problem ? , if your uping the power don't go to daft ie 21 ftlb hw80 are very sweet at about 17 - 18 ftlb in .22
ATB JOHN
hw 100, hw80 mk1, smk xs20, bsa scorpion pistol, feinwerkbau 127, hw99s