If Rythmik still offers a 10% discount on multiples, you should just be able to get a pair of the E15 subs for $2000. The only difference between the E15 and F15 is the size of the enclosure, and therefore a minute difference in output.
Whether or not the F12 would provide enough output for you is also partially determined by how loud you listen. The F12 standard uses the Rythmik aluminum cone woofer, the F12G the GR Research paper cone woofer. There is no F15G.