None of this had to do with your upgrade to SP2. Nor did SP2 update your drive's firmware for you. That's something only you can do. Anyway, the tested burn speed is what appears as speed selections. Whoever made your drive tested the Maxell 8x DVD and decided it was fit to burn at 4x, 6x, and 8x. 2x may not have done well with the faster write strategy of the disc.
Were you using the same media this whole time? If you, say, had a spindle of 4x media you used when you had SP1 then upgraded to SP2 as well as changed media... It's all about the media.