I don't like relying on math to prove you wrong, but I checked the Snake frame data.
http://www.smashboards.com/showthread.php?t=258332&highlight=frame+data
Ledge Attack(under 100%)
http://img13.imageshack.us/img13/976...ttackquick.gif
Invincibility Frames begin at frame - 1
Duration - 21
Base Damage: - 8/6
Hitbox comes out at frame – All 24
Total frames - 56
Ledge Attack(over 100%)
http://img696.imageshack.us/img696/3...attackslow.gif
Invincibility Frames begin at frame - 1
Duration - 43
Base Damage: - 10/9/8
Hitbox comes out at frame – All 41
Total frames - 70
The shieldstun formula is basically damage divided by 3. So the formula for frame disadvantage on block is total duration - first hitbox frame - shieldstun. <100% ledge attack has 2 frames of shieldstun or so. >100% ledge attack has 3 frames of shieldstun. Assuming you round down iirc.
G&W's Up-smash hits on frame 24. Perfectly timed out of shield, there is no shield drop lag. F-smash out of shield is 24 frames. D-smash out of shield is 22 frames. Powershielding is irrelevant if you're Up-smashing out of shield since shieldstun still occurs on powershield.
Snakes <100% ledge attack is
-30 on block
Snakes >100% ledge attack is
-26 on block
Basically in short, my math AND my personal experience in this matchup (my friend Crackle is a legit Snake main, I could just bring him here and have him vouch for me that it works) are both contradictory with what you're suggesting. I think you just aren't timing it correctly. If I saw a video of what you did, I would be able to confirm whether or not you timed it correctly.
Hell, I've Up-smashed out of shield with LUCAS against ledge attacks. His Up-smash is frame 28.