I cant remember the size altenator in my Focus is, but I doubt it is the size of yours and I get very minimal dimming at all.
Redtop up front
Powemaster 5100 in back
All 0/1g Knu Konceptz wiring, no big 3.........and running 1700rms.
I suggest in your case doing the big 3, and a couple batteries, one replacing your stock and then one in the back, just make sure they are not wet like the one guy said.
Check the classfieds here and see what you can find. Got 4 batteries over the years from this site.......alll with good results.
Paid 80 for my redtop and 130 for my powermaster.
Good luck!