"It must be hard to get your mind out of the gutter when the rest of your body is attached"
"IQ~wise: You'd have to jump up to touch bottom!"
"IQ~wise: You'd give a sack of hammers a run for its money."
"Your head is so far up your backside, if you'd f*rt, you'd blow your brains out!"
"The boy was born downhill and has been losing ground ever since"
"If your brains were Dynamite, you couldn't blow your nose!"
Jack.
How about: "I'm trying to see it from your point of view, but I can't get my head that far up my @$$."