The KB is your friend my man. Building mounted glories and Legends capes increase the xp per plank, and you get your original items back. With legends capes you get 100xp per plank (761gp today). (That is 10 more than they normally give) Nearly double the 60 per oak plank (422gp today). Cost: Teak: 7.61 gold per xp Oak: 7.03 gold per xp Yes...teaks are clearly more, but I am not going to try and convince you how much faster they are to make. If you can't try it for yourself, then I recommend you stick to oaks.
The best thing to alch is: NOTHING Grab a stop watch, and cast superheat on iron ore (not mith, steel, or above...) 27 casts and bank, repeat. The alch any of the items listed in this thread. Compare the results.
D'OH! I forgot about the hammer, saw, and money. I will still take the Pepsi Challenge any day though. One inventory gets you 2400 xp, compared to 2 inventories of oak at the same xp. You lose more money that way, but not much more, especially if you are not inpatient and buy your teaks at min. I use the time saved to do more profitable things, ending up with an overall higher xp and gp per hours played. It is all about economical use of your time.
Grab a stop watch and do the math for yourself. It is easy to confirm. (Also, there is no method to alch faster.) Yes, the numbers were conservative. I regularly push well over 100k mage xp an hour with superheat.
You are also forgetting that you can make 9 per invent of teak (for 2700xp) and only 3 with oaks(1200xp). Even if oak doors were twice as fast to make as legends capes (they aren't), you still come out behind XP wise. You can concentrate on making money for money's sake and con for con XP and come out ahead with teaks any way you look at it.
A long neglected spell, superheat kicks the crap out of high alch. You lose less money and the XP per hour is higher. One thing that always amazes me is why more people don't grab a stopwatch and time things more often. There are no items out of the GE that can be purchased and cost less gp per mage xp than the cost of superheating iron. On top of that you are also gaining some serious smithing xp.
Have you though about training with teak? Legends cape: 3 teak + 1 cape of legends - 300xp 1 teak = 100xp 1 teak plank is currently 780 in the GE Cost per xp: 7.8gp Xp per trip: 2700 Oak Door: 10 Oak plank - 600xp 1 oak plank = 60xp 1 oak plank is currently 436 in the GE Cost per xp: 7.3gp Xp per trip: 1200 Finance wise you come out a small bit behind with teaks, but it only takes half the time, and if you can buy planks at the minimum price (I always do), teaks are better gp/xp as well.