I've been skating a 8.25 deck with 8.5 axle trucks for the longest time. I recently started skating 8.5 decks and I'm liking it. Would moving up to 8.75 axle Independents make a noticable difference for better or worse?
For the most part, it will be the same upgrade, but some people find that the extra width and weight can be a little too much to deal with, compared to a smaller setup, even with the same offset of wheels as their other boards.
I am more of the thinking that 149s work well on 8.38 and 8.5 with extra washers on the inside, then 159s on 8.6 or 8.75 boards, again adjusting washers to suit.
Some people won't go over 149s even on bigger boards as they just can't handle the extra truck width, but others swear by the 159s on 8.5 type setup, only one washer on the inside, for one guy I know who likes that.
Washers can make a big enough difference, that you may not need to go up truck sizes, but even just to sit the board grip side down, loosen wheel nuts almost all the way off and push the wheels right out to have a look at it and see if it looks too wide, just as a reference. Don't skate it like that, just look at it. That can help to see if that is what you want.
Yesterday at the shop, someone said his trucks were too narrow - having one washer on the inside, one on the outside, as per how whoever had set it up for him. Then when I did a little changing up to add some more for a total of three washers on the inside, none outside, they were wide enough so he didn't want / need wider trucks after all.
Hope that helps.