I've used a Glock 27 sub compact in .40 as a duty backup pistol in a BUG holster sowed on a a BP vest. It shoots fairly well but you lose some precision due to the shorter sight radius. The shorter height means the bottom of the mag is still on the palm of your hand instead of below it. If you're just using it as point and shoot or shorter range where you shoot just over your sights at 15 yards or less, it's pretty effective. If you're shooting past 15 yards, you notice an extra half second or so to aim for precision shooting.
I've also carried it CCW in a gun bag and the bag almost dwarfs the pistol.

I haven't shot a .357 SIG so can't comment on it. I've noticed the .40 is a little hotter than a 9mm but it's really not that bad. Currently, I'm carrying a compact .45 (G30SF) which is compact but points and shoots well. It's not a sub-compact but isn't much bigger than one and it's a bigger bullet.