I think an extension should be included in the weight limit. The extension becomes part of the cue hitting the CB. Some players play with the extension on for all shots. Then it just becomes a longer cue (there is no maximum length) but should still have to meet the weight maximum. Otherwise, what is an extension? If you wanted to use a 30 oz. cue, just do it by having a 3-piece cue and call the back part an extension.
But I don't know why the equipment specs put a maximum on cue weight. Does a cue over 25 ounces confer any advantage to anyone other than André the Giant?
I'd like my opponents to use 3-pounders.