If my calculations are correct stating from level 52 youll be able to train at 5.6m xp per hour, that's just plain stupid in my eyes.
Betting my ass you aren't taking the time it takes to cut logs into account.
nobody takes into account the time it takes to chop magics for fletching either
Magic logs you can buy in a relatively short time if not instantly, because they are a common drop from high-level monsters and the most popular way of training WC after 99. So even if you take that time into account, it won't affect the XP rates that much.
Mahogany logs on the other hand are far from being the best woodcutting XP in the game and they will only have one use it the future, construction. This means there won't be a lot of them coming into the game unless Thomy decides to go ham on monsters drops. This means buying or gathering them will take ages. Making planks might even result into money loss if nobody buys them, which means making planks can actually be risky after the release hype of construction. You won't have any risk of losing money while spinning flax.
Buying 1k toadflaxes at over 2x the commonly traded price took me 3 weeks so I could get my herblore up 5 levels for 99. Supplies in this game are rare enough as they are, no need to make them even harder to get.