Mods normally slowdown Skyrim but I guess that depends on the mods. Just replacing some graphics wouldn't but adding new gameplay functionality would.
I think Skyrim is clearly a title paid for by Nintendo so the Switch appears to have wider support. I can't imagine it selling well so I suspect this is going to be a by the numbers conversion with Bethedsa not expecting to reap much in sales once launched.
It will be pretty much the 360 version visually with minor visual upgrades with the original paid for DLC included. 720p 30fps portable maybe 900p 30fps docked.
I suspect it will be shoe-horned into a 8GB cartridge or may get the full 16GB cartridge size and be sold at £60 here in the uk. I honestly can't imagine many people buying it.
What would be great that could transform the game is a huge update in feedback, I.e. sword fighting, fireballs, a twang in your hand as you shoot an arrow. This is what I'm hoping for that will justify the game and pricing but if it doesn't provide such an enhanced experience I can't see it doing well at all.