The only effect is that your industrial (military and social) efficiency on the planet type in question is improved by the stated amount.  That is, 0% industry at level 0 (say, if you invade), 50% industry & the ability to colonize at level 1, and full industry at level 2.

This is multiplicative with all other industrial modifiers, so it's exact, barring truncation.

There is no other effect.

Not useless.  Taxes, influence, and research are still at 100%.  So is food, for whatever that may be worth.

At last check, anyway.

Actually, the influence from the planet proper may be affected (I've never looked into that), but influence from population isn't.

  You can still rush-buy improvements;  useful after you have invaded a planet and it costs only a few BC to finish a part-built tile (often research buildings in my metaverse games) ,  and for rush-buying farming (xeno-farming is only about 500BC from new) if you want the planet's population to continue growing.

As I normally play as Korath in crowded small and medium galaxies, I often have at least a couple Toxic colonies, noting that I start with the basic toxic atmos tech. If I research the advanced tech, will these colonies then have 100% efficient production?

In my ongoing game, I am gearing up for an inevitable war with the Terrans and will hopefully be sporing their planets, turning them into my toxic colonies. Perhaps I want to research the advanced toxic tech prior to sporing so that these new colonies will have 100% production?