I do feel like lowering the AC of goblins is an overall good idea.
With an AC of 15 your entire party can miss all of their attacks in an entire round, which can be very boring in single player.
However a flat double of HP is a bit weird - I'd say scale it down to 10-ish, which should allow a fighter to kill them on high rolls with one hit, as well as allow Rogues Sneak attack to shine.
Goblins aren't that hard to hit with a 15 AC, and when you do hit you usually kill them in a single blow which is the way it should feel. Goblins are weak monsters, my fighter shouldn't have to hit one 3 times to kill it, they should be chopping them to pieces on just about every hit, not just on crits or max damage rolls.
