i agree w/ LBCrew in that as the surf gets better, the emphasis switches to controlling speed rather than generating it, & for that, i still prefer a thruster set up. i did recently have a board made by brian wynn w/ the idea of experimenting w/ quads in better surf in mind, so we'll see how that plays out...can always run it as a tri fin.
fin-wise, i'm a bigger guy, so i've never used the k2.1's, but the quad rears from that set are my go-to quad rears for most boards. my favorite high performance quad set up is G-AM fronts w/ K2.1 quad rears. i just picked up a set of the MF-1's & am stoked to try them out the next time we actually have waves. i tried a "nubster" type fin in my hp quad groveler & was unimpressed...added lots of speed & hold, but stiffened the board up beyond belief. i think that it's something that's better suited to larger, steeper, or more powerful surf rather than waist high wind swell. i'll keep it in mind for my new wynn, but it'll never touch my groveler again.
the cool thing about the kelly fin sets is that the thruster rear fin is the same on both sets, so even if you just buy the k2.1 quad set, you can still try it as a tri b/c you have the k3's.