I have heard great thinghs about the Denton 120, there is a nice contribution
http://www.spearboard.com/showthread.php?t=158301 of a recent trip in Panam where they used Denton 120 with 8mm shafts, twin floppers but with standard band lenghts to shoot hugee garoupas and cubeiras...if you google videocean they have also conducted pool tests of several guns and the Denton 120 was the fastest speed m/s with 8mm 2 16mm@ 400x...more than with 3 14mm and more than Abellan roller with single 19mm...very interesting gun...
Have you changed the settings or tried the 8mm shaft? I think probably the original set up may be the best in flexibility to diferent situation with perhaps some 8mm with slip tip for deeper reef or ligt BW?