Reading the install instructions on the kits, you don't need the bracket if you just want the splitter, so, if you only wanted the splitter, and not doing the laguna seca portion, you don't need it. Thus, go for the cheaper kit and be done with it.
I would like to have the splitter, but not $600+ worth. I don't blame the distributors for that price, Ford sets the price they sell it for to them. AM and everyone else has to pay that price for it FORD sets. I have to admit, even if AM, (I only use them as an example since they are my preferred vendor), could sell it for $400, I am not sure if I like it that much to pay that price though. (but the BOSS splitter would be a nice finishing touch to go along with the roush side and rear splitters)
2013 GT 6 spd, GT500 mufflers, BOSS quad exhaust, Roush CAI, Barton shifter & 2 post bracket
CS GT-40 Wheels, Hotchkis sway bars, upper and lower control arms, springs (1.1/1.6 drop) and adj panhard bar KONI yellow struts and shocks J&M caster/camber plates,
BMR Upper control arm mount and lower control arm brackets.