yes I did! writing killed me (and speaking once)! English is not my first language and I needed seven in each band for my position! I always had 7.5 as a overall band, so this was pretty stupid to do and redo that test! Actually, I worked with a teacher for 10 jours between test 3 and test 4. She explained me how to write "british" test and it worked. And I practice again and again... Got 7.5 at the last writing test!
If you think you need 8 to claim points on the visa, maybe you should make sure you don't have enough points with the others parts (age, experience, qualifications...). Maybe an migrant agent could tell you. I remembered our migrant agent told us before we signed any contract with her that I just needed 7 in IELTS and not 8, so she knew while studying our case how many points I already had.
Regarding the test itself, it took me a long time because the tests are fully booked in France and you have to wait two months before each session. A friend here in Australia registered to 6 sessions in one month and went to 4 of them until she got the results she expected. A waste of money of course but at least, she managed to get the results she wanted.
I am sure you will get there... think about a short private course with an experienced teacher perhaps?