Well Jove, nobody has posted on this thread so I'll pitch in.
You can say with good assurance that Thorium, Cerium and Lanthanum as well as Tri-mix Electrodes all have about the same current carrying capacity. What seperates them is how they perform when overheated.
Thorium tends to grow irregular nodules on the tip and transfer particles across the arc to contaminate the weld when overheated.
Cerium and Lanthanum electrodes are supposed to have better integrity. This means that when the tip starts to melt that it will form a hemispherical ball on the end rather than spit into the puddle.
Lots of folks use thorium to weld aluminum (even though the textbooks and manufacturers generally advise otherwise). The trick is not to overheat the tungsten. A full length tungsten will hold more heat than a stub. Also a water cooled torch will allow you to weld at much higher amperages than air cooled torches.
Zirconium has a lower melting point than the above mentioned electrode types but is still far superior to pure tungsten.
Technique:
Sounds like your doing fine in the outside corners so I'll adress fillets. With a little extra surface prep (removing oxides immedatly prior to welding) you will be able to set your balence control closer to max. penetration. This will help put more heat on the work and less on the tungsten. A steep tapered tip with a blunt end will produce a more directional arc than a rounded tungsten (stay away from long slim tapers). Keeping your electrode perfectly clean and very tight in the fillet will help keep the profile small. When you dip your wick stop and clean it. If you have an inverter powersource, increasing the frequency to between 180 and 220 hertz will also constrict the arc. Again. If you can fit the long end cap into your work, a full length electrode will allow higher amps. A water cooled 20 series 250 amp torch can get into some pretty tight spots.
Sometimes its hard to convince the boss to spend more money on things like tungsten. So you may have to get your edge by really focusing on control of process and technique.
Let us know how it comes out.
If you are working with conventional, non inverter equipment, and require a "balled electrode", zirconiated would be your best choice.
It has the current capacity of 2% and some of the rare earth's.
I run both a conventional squarewave and an AC inverter. After much experimentation on aluminum-
Zirconiated on conventional tig equipment.
1.5 Lanthanated on latest inverter AC tig (Miller aerowave, dynasty-Thermal Arc GTSW, etc).
FWIW
Good Luck
brande