Ok, first off, have you got the book? If not, you need to buy it. The fact that you asked 'Should i be putting my joints through this much stress' makes me think that you haven't. In fact, having reread your post and seen 'Should i keep the same exercises but drop some poundage and up the reps' I know for sure that you haven't read it, so do yourself a favour and order it today.
Secondly, yes, linear progression with weights gets HARD. Though 80kg is not hard. It only seems so because you weigh 150lb. Read the book, and start eating like a 200lb muscular beast, start putting on some decent muscle and size, and you'll find 80kg squats become suspiciously easy.