I honestly think Anthony Johnson has the edge over GSP in the striking department.
GSP was able to out jab Koscheck, but Anthony Johnson actually rocked Kosckeck a few times, causing Kos to go for takedowns. If Johnson had sub defense, I somewhat doubt Kos would have been able to hold him down for three rds, and he probably could've won a decision or possibly gottan a Knock out.
People like to complain about the superfight match-up between GSP and Silva, saying Silva's size advantage is too large
for it to be a fair fight. Anthony Johnson cuts from 230, so he obviously has a bigger frame than GSP. AJ also has the advantage when it comes to having raw power. I think a BJJ defense Anthony Johnson could probably fend off submissions and possibly get a KO over GSP.
Ofcoarse, this is just a complete assumtion...