I don't usually take the time to post but I am gonna help you out here : ) Good choice in choosing the Darkstar over the Anagram. The Darkstar has a better core, sidecut shape, and base, all more conducive for all mountain riding as compared with the Anagram. The Anagram is relatively SOFT and is more similar to the WWW, in fact it's almost the same deck minus a few tweaks here and there such as blunted and tapered tips, and a tapered center on the WWW. At your weight and for the kind of riding you're looking to do, a soft board like the Anagram is not what you're looking for.
If you loved the Believer you will enjoy the Darkstar, there are some tech differences between the two but nothing that is gonna blow you outta the water. As for sizing..i'm under the assumption that
10.5 UK is 12 US? Your Believer at 25.3 is too narrow for your boots, which you know. Unless your boot has an extremely large outer profile, ie, some DC boots, Northwave etc.. you should be able to get away with a board in the lower 26 ww range. You want to go as small on the ww as you can, without having too much overhang, so that you don't loose any edge response. With your average size 12 Burton or Salomon boot (or something that sizes similar) I would say 26.1 - 26.3 would be ideal, which corresponds to the 157w and 160w Darkstars respectively. The 163w at 26.5 is overkill for your boot size and, more so than the length of the board, you will notice how that affects the ride and performance. Considering you are looking at a mid-flexing all mountain park board I would assume you want something that can go everywhere but still has some dork around fun to it. If the place you ride doesn't see a whole lot of powder days, you are riding mostly groomers, and like to dick around here and there with some flatland tricks, lap or two in the park maybe, I would consider going with the 157w. Tricks really not something you're concerned with, or maybe you do get a lot of snow, go with the 160w. Either way you will be happy. If you rode the Believer in a 161 and that felt good..go with the 160w. Nay to the 163. Hope this helps. Oh and about the Anagrams "catch free edges"..the amount of camber is just less than their other boards with "traditional camber"..more of a gimmick than anything and yes you can still catch an edge if you don't know what you're doing.