Sig recommends the Avistar .46 2 stroke for their LT-40' The Avistar is one of the highest output .46 size out there, not a .46la. Most 60 size trainers want a powerful .60. FOr instance the Alpha 60 recommends the Evolutiom .61 not a bushed anemic 60 size.
Can you fly a 60 size trainer with a good .46? Sure. Can you learn to fly that way? again, sure but your margin of error is lessened, No room for growth... you'll still need to buy a 60 for anything else you fly in a 60 size ( tiger 60, 4* 60, At-6. etc.) Can you have fun flying a 60 size powered by a .40? If you like flying a kite. Everybody has a Piper cub in their hanger, but how many of those get flown regularly? Very few and only in lazy circles on calm days. The exceptions are the overpowered Cubs which just reinforces my opinion. Put 40s on 40 and 60s on 60s and you'll be just fine
That being said, I'ld rather fly a telemastion giant with a .25 even if I had to throw it off a cliff than not fly anytrhing! Have fun!!!