There is no fury gain of using FF over SB, in fact you're losing fury because more LTs means less fillers.
If your SBs are 2.03s, then your haste is 2.5s/2.03s=1.2315... That makes your GCD 1.5s/1.2315...=1.218s. Fury per second while casting SBs is 25f/2,03s=12.315270935960591133004926108374f/s. Fury per second while casting FFs is 15f/1.218s=12.315270935960591133004926108374f/s.
No I didn't just copy the same number into each calculations, plot them into a calculator yourself and you'll find they're identical. You'll see it easier in an unhasted scenario:
25f/2.5s=10f/s
15f/1.5s=10f/s