I have a 2005 specialized Sequoia, currently with a Shimano Sora triple. As it turns out, I _never_ use the 30 chainring. So, I was wondering, why not getting rid of it. My questions for the experts would be:
1) Am I thinking of doing something really dumb?
2) How can I rig the Sora front derailer to “click” just twice?

Thank you in advance,

In other words, you want to limit the crank to just the 53 and 42 rings?

As a first step... I also would not mind losing the 42 ring (steel in the Sora) in favor of an aluminum one (maybe a 39-40T? I guess I'd have to figure out if I'd have "gaps" when shifting from one chainring to the other)...

Yes, you can put a 39 instead in the 42 and get rid of the 30. You will have to adjust the internal limit screw of the derailleur to prevent it to move the chain down from the 39 ring. That's easy to do.
Another option would be to buy a new crankset (Ultegra double) and sell the Sora triple.

Your Sora shifter will always click three times. If you set the limit screw on your front derailleur to only drop to the middle ring, the third click just won't do anything.

IMHO, the advantages of a compact double are as follows:

1. Less weight. But with the Sequoia, you have a virtual smorgasbord of possible upgrades to save weight if that's your fetish.

2. A double does shift a little smoother than a triple. But this doesn't really become a reality until you switch to a short cage derailleur in the back and move the chainline in. If you still use the Sora shifters, you could still be running into trim issues.

3. Compact doubles are great if you already are running a 53-39 double and want better gears for climbing but don't want the expense of "upgrading" to a triple. Not your situation.

I'd keep the triple. You can put a smaller middle ring on the bike. You'll use granny ring even less but your ratios will be closer. I had Sora with the same gearing as you have on my Trek 1000. When I swapped the 42t for a 39t I found I didn't have a big gap right where I liked to ride.

Unless you're changing to a shorter bottom bracket spindle, you won't be gaining anything by removing the inner ring, just losing the ability to use it. If you want the benefit of a triple, you'll want to at least change the bottom bracket, and as mentioned above, maybe the cranks.

Thanks to all for the great responses. I wonder if this is indeed worth the trouble (I'm not a weight fetishist but I do like to, ahem, "improve" things

Happy New Year to all!

IMO, it's not worth doing.