Yarrussky is awesome... Better than Dr G. in my opinion. Had him in year 10 and now in yr12 spesh. His tests are great prep for exams, don't lose confidence with your scores (if they are poor). Wish I had him for GMA too 
I do miss Yarrusky's tests. Had him last year but Dr G. this year. Last year Yarrusky's test were super long and he allowed you a double period to do it. But when you finish all his questions, you just have the best feeling of achievement.
Just did my first Dr G. test today and I was destroyed. Couldn't finish half of it, yet I was able to finish every single Yaruskky test.
Back on topic: There are two types of people that do Further over Specialist: 1. The people that actually can't do Specialist because their minds cannot comprehend the concepts behind the subject. 2. The people that don't excel in Specialist but can cope, however they believe they will get a higher score or similiar score yet can put in less effort.