I don't quite agree. GSP is a great MMA wrestler but his takedowns are based more on timing than brute strength & power. Against everyone he's faced so far it works great since GSP has better speed & timing than all of his opponents to date.
Against Anderson Silva, well, you got a problem. Anderson has better timing than everyone else, and it's a pretty big gap, plus he's just as fast if not faster than everyone. If you're Chael Sonnen and you just run through people and drag them to the ground it's not as big a deal, but that's not the kind of wrestler GSP is. He's not relentless like Chael, he times things, gets his opponent to commit or hesitate and that's when he takes them down. Not going to be easy against Anderson, and there's a decent chance he'll get faked out or countered by Anderson and eat a knee.
Is it impossible for GSP to take Anderson down? No. But it's going to be harder and more risky than most people think.