That's silly. The LA's have more power by far than implied by Fastsky.
can barely get a trainer off the ground
If you only know how to take off by hauling hard back on the elevator and climbing nearly straight up, then yeah. But if you know how to fly, using the wing for lift, the .40 LA has plenty of power for a trainer. I've flown everything from an LT-40 to a superstar and my airvista on that engine, and it's fine. And I fly from a grass field that is sometimes not mowed very well. But I still have no problems. Now, you want to use an approprate prop of course. I like the APC 10x5 on the OS .40 LA. An 11x4 isn't bad either, though I mow too much grass with the 11" prop.
It's true that the LA's aren't very powerful for their displacement. But they have enough power to learn to fly, enough power to loop a trainer from level flight and do other basic aerobatics. You won't be able to climb vertically out of sight, but that's not a bad thing when learning to fly.
If you plan on taking the engine out and using it in a second plane, then it's probibly not the best choice. But a little secret, most guys actually don't do this. They keep the trainer flying while building the second plane, and end up buying a new engine anyway.
In the end, it's really not a big deal though.