I'm talking about the one that came with the KL+ Veradas with 'premium audio' on the front.

Will they sound decent?

you would be better off saving a bit of money and getting your self a decent head unit and getting a 4 or 2 channel amp to run them.

You won't hear much of a difference, the stock splits are pretty good for what they are. You'll benefit more from a new headunit, and then again with an amp as Perry suggested. But then if you're getting an amp, you might as well upgrade the splits......and it begins! lol.

Start with the head unit.
If you amp the stock speakers then there is a good chance you will blow them, Ive seen a few of the old 15w speakers that came in early 3rd gens completly inverted cos they were amped and volume run up.