Hi friend!
Don't worry about the scores.Try to use the mock exam as a learning tool and learn more from your mistakes.Dan's exams are really good.He covers some stuff that are not asked in the exam but your java concepts become clear after taking his exam(all 27) and I'm sure if you score above 90% in marcus Green's exam ,you sure can score the same in the real exam.The difficulty level is almost the sam.But none of the Thread and Garbage collection questions in the exam match any of the mock exams.So try to do more questions and practice more on your own in these 2 topics.All the very best.All the exams can be found in this site(including Dan's and Marcus')
AllMockExams And Sun's epractice exams are supposed to be good but I would recommend not to spend so much money on that unless you really want to.There are others in this site who have scored 100% without spending that extra money.But do as many mock exams as possible and it will definitely help.Good luck.I hope that helps.