hi there,
definitely do bristol, has the most up to date and accurate database, there was only 3 or 4 q's i had not already seen when i did my jaa's, try to get the whole database in before your exams but don't over-cram. i did all of human perf. in one day and it definitely had a limited benefit. the search tool on bristol is useful for finding those tricky q's as well so you don't have to do the ones that you find easier over and over again. a lot of this is getting used to the style of question thats going to be asked, some of the wording is terrible and once you have the technique sorted you'll be fine. where bristol comes into its own is the simple "fact answers" where there is no calculation...you either know it or you don't and if you know most of the bristol answers then you will be fine. and trust me, its far easier memorizing 900 bristol answers than a 90,000 word textbook...!!!
hope this was usefull...