The multiplier shouldn't be lowered just because players would max it way too fast. It will be done anyway, no matter what the multiplier is.
You can already max most skills in 24 hours (for each skill). Getting 99s has always been easy.
Your proposed lower multiplier would just ruin the balance between skills, especially when it comes to long-term skilling (200/250m xp goals).
With member bonus it already takes 192343 mahogany planks to get from 99 to 250m construction (96172 planks if you use double xp as well), so it already is a great material sink.
Cash is also sinked via sawmill, butler fees (if we have them) etc. Also building things like gilded altars, which aren't part of powerlevelling but still quite an integral reason for construction's existence, sink quite a lot of cash.
Also as far as "make plank" spell is concerned, its rework is not required (except making it work on oaks, teaks and mahoganies as well). I'm quite fine with it making 1 plank per cast, since it takes about ~38 seconds to make a full trip to sawmill with 26 logs.
Currently you make up to 2.5k planks per hour via sawmill (my average was like 2463 planks/h). As I've made 30k logs into planks already, I can say it's not that bad.
Since you guys think it's way too fast, this should be a point, where we restrain our need for comfort by not implementing full-inventory "make plank" spell. This way sawmill stays as an essential part of the skill, which would turn plank making into a very profitable money making method (richer players are too lazy for manual labour, so they'd gladly buy the planks directly).
Ameer also mentioned Brawlings gloves: I don't think this skill should have these. Considering the current plan for xp rates, I'd follow fletching's and crafting's lead in not having major xp boosts.
Of course we could have exactly the opposite by having lower multipliers, but compensating it with multiple items that can boost the xp, resulting in the need of less materials (this should be done to RC atm). However, I can't think of any proper places to place these xp boosting rewards. There's no such minigame that would fit with the skill.
In my opinion, the 8x multiplier is perfectly fine, especially if you consider how much materials, cash and time you need.