I think the key "taxing" part is the run, since out of the three, it's the only full weight bearing activity.
The commentators of triathlon always note that the run is the hardest part, not becoz it's last, but becoz it's the one discipline where you are not aided by anything.
Swimming, you are aided by water, you weigh about 10% of your body weight - so it's not weight bearing
Cycling, you are aided by sitting down, and momentum, not sure about the %, but obviously, sitting on a sit, with momentum of 40km/h, in terms of how "taxing" it is on your body, its definately less than running (and more than swim). However, the bike takes 4-5 hours, and the run 3ish, so it's hard to compare the overall effect.
I know most forum posters hate LA, but I wish him good luck at Kona. I think if he can pull of the swim ten minutes down on the top guys, catch up on the bike, and as he has said "be fresher" for the run, he could well pull of a top 10 result. Given how large the time diff is between 1 and 10, i think he can do this.
Given he has run 3 sub 2:50 marathons, i'd say he could run a 3hr 42k at Kona, especially after being so well trained for the bike. The swim is the key unkown part for him. I guess 2010 is his last TDF, so he'll have 15 months to train for KONA, and i'm pretty sure he can qualify with that amount of time, so it wont be a free entry as many suggest