smeg, I think some misunderstandings developed because of my understatement of the tech situation.
At the initial 10k range, an 8HS BFC with a twin turret will have a 50% hit rate with four shots (2) while a 16HS BFC with a single turret will have a 75% hit rate with two shots (1.5). If the BFC had a 20k base range, you'd be looking at a 75% hit rate with four shots (3) versus an 87.5% with two (1.75).
The "four shots" comments here were referencing the fact that, with the RoF tech at two, two GCs will get off a combined total of four shots, *not* the requirement that he have researched RoF 4. The bit in parenthesis is simply the expected number of hits (I assumed that would've been fairly easy to deduce, but I figured I'd state it clearly just to make sure).
Not sure where you got the 7% increase in HS use, as my numbers were significantly smaller (My numbers showed a 0.16HS increase using the 8HS BFC). Using a 40% gear figure (Standard if you're matching turret and BFC tracking techs), a single GC turret would be 8.4 and a twin would be 16.56. Take off the 8HS from the reduced BFC and you're left with a 0.16HS increase, or, in percentage terms, a 0.65% HS increase.
Largely agree with your closing paragraph, although BFC and turret tracking speeds are high on the list as well (The former a bit more than the latter as BFCs have a hard cap, unlike turrets).