I think squam is way too big to be effected much by any spring cooling. I would guess the lake stratifies, and the top layer will be come too warm to hold trout for anything more then a short trip to feed. Besides there's no trout in that lake during the summer as far as I can tell.
I do fish a lake which is mostly spring fed, and although it is much much smaller than squam, I have caught rainbows on sinking fly line there in the middle of august when the water temp is well over 70. I'm usually trolling pretty fast, and I can't imagine the full sink is getting past 8' down.