EDITS everywhere:
The drag engendered by thrusters during turns with the outside fin out is a function of the aft-and-in placement of the rear fin, more so the aft placement, I suppose, which gives resultant leverage on the rear fin, which results in resistance on the high pressure side and drag on the low pressure side (of that fin.) Also the differential of angle between the stringer-line mounting of the center fin and the toed-in rail fin. This all affords, of course, stability, an opportunity to push off that resistance, and a lot of feedback from that control surface. Also, to add clarity, and perhaps to state the obvious, the rail fin is stabilized in an effective positive AOA by the rear fin as the hull banks and turns hard, in other words the stabilizing fin maintains the rail fin’s (Bernoullian/Newtonian) effective AOA (of course,) which in turn augments the rail’s (primary?) hold. The rail fin and rail want to blow the rear fin out, if it’s too small, say, but if it’s correctly sized, and everything is correctly done, the thing holds in, but with a pretty sizeable coefficient of drag. Again this is because of the leverage given by the aft placement and the different angles of rail and center fin. The inward positioning of a rearward fin seems to me less optimal than a more corner-oriented position, partly at least because of the mass of the water 4 or 5 inches under, versus 3 or 2 inches under (when banked over.)
There is less drag on quads because the rearward fin is typically not positioned very far aft of the primary rail fin, so there’s less levering against the planing flow over the rear fin, which levering on a thruster causes high AOAs and drag off the rear fin. The fulcrum of which, BTW, is the rail fin and rail curve adjacent. Rail forward of the rail fin is the other end of the teeter-totter.
Quads are loose, and lack drive, some say. They mean there is a lack of leverage and stability, I reckon. Without the longer lever a lengthier fin cluster gives, the pair of fins on a rail are less stabilized in their track.
I would be very interested to know what results if you put those quad rear fins 4 inches further back toward the corners of the tail. Maybe by sinking the rearward fins deeper in the water, you could run smaller fins than with the std. quad configuration and still get stability and less drag than the thruster. Like a Griff, but without the center fin, with the rear fins a bit further toward the corners. I wonder how small they could be. I saw some sidebites mounted (parallel, I think) at the aft corners of an old single fin that someone said you could ride anywhere on the wave…
shrug
grreg