Hi Jon,
I have a set of Biomaster GT 8000 and have tried the black Tackle Box type conversion on them. The conversion replaces the clutch knob, and has a metal lever that you lift up to free the spool. However, I don't find it a particular advantage over losening the clutch, on this make of reel.
The clutch on the Shimano' s is only a few clicks from locked up to free running, and thus you'll find it very hard to adjust the bait runner conversion so that it a) gives a free spool on a take and b) is tight enough to play a fish once you've made contact.
I stopped using mine for that reason.
Conversely on the Diawa Emblems, which I also use, the conversion works brilliantly and avoids the endless winding once you've struck, and the old problem of losing the clutch knob.
So I'd say on the Biomasters don't bother they are too much hassle, but on the Emblems with their longer thread they are a great accessory.
cheers
Gareth