Thanks Guys for all the comments. BDMicarta, your thoughts on the detent may be spot on. I also think that there is a mass factor in there as the larger blade of the Tenacious has greater inertia once the opening has started.
I've recently ordered the Gayle Bradley, and whilst the Spydie hole isn't the biggest and there isn't complete access to it - it seems to be reasonably fast to the practiced hand from what I can tell. (I should get mine in a day or two!) We drill handgun drawing, etc. so my thinking is to drill blade opening and to groove a consistent approach. So I guess I'm rather focused on using the hole as opposed to a flipper/wave option, although there is no denying that they work well. (My ZT's assisted or not, fly open).
I have several other makes of knives but my Spydies just seem to fit the bill in terms of carry and as a result feature in my EDC far more often than anything else.
Cheers all!
Ben