"by as much as X percent" is a way of discussing uncertainty. The author doesn't want to guarantee a particular number, and prefers to merely suggest that maybe this will be the value, and probably not more.
does it mean "by up to 8 percent"?
Yes. Very close. As noted in earlier comments "The as much as X construction is more or less equivalent to up to X, but imho that second alternative has slightly stronger implications the it won't be higher than X."
does it mean "by 8 percent"?
No. That would be stating an exact number.
Occasionally an advertisement will say "by as much as X percent, or even more!", which if carefully analyzed means they are saying absolutely nothing, and the number could be anything.