Most of the new stuff should be on Serebii, but it's alot less detailed than the specific discussion threads in Smogon. For example, the description for Magic Mirror on Serebii neglects to mention that it can reflect Stealth Rock (and other entry hazards) and Taunt, so if I switch Espeon/Xatu in on a suicide lead when they SR or Taunt, not only am I not taunted or have SR on my side of the field, they instead would have SR set up on their own side, or be taunted.
As to how Ditto's stats work, transform and eccentric copy every stat of the transformed Pokemon except for HP, which is why EVs in any stat of Ditto's other than HP is a waste. It even copies some item buffs like the Choice items, since they are buffs applied to your stats directly. Other items like Life Orb and Charcoal and the like will not be copied because they apply to your attacks/attack BP rather than your stats.
So let's say you have a normal 4/252/252 Jolly Salamence with 1 DD under it's belt and a Life Orb. It's stats would look something like...
HP: 307
Attack: 553 (369 + 1.5x applied from Dragon Dance)
Defense: 196
Special Attack: 230
Special Defense: 196
Speed: 492 (328 + 1.5x applied from Dragon Dance)
Once Ditto switches in, it copies all of those stats except for HP. Peroid. Doesn't matter if he doesn't have a 4/252/252 spread or that he didn't DD himself, he just takes those stats. That's why no EVs other than HP matters, and that's why there's no reason not to just max out Ditto's HP EVs. He doesn't copy items though unless the item has a permanent effect on his opponent's stats. If Salamence had a Salac Berry for whatever reason instead of a Life Orb, and it activated, it would copy the stat increase from that as well.
Now let's look at Ditto's stats when it automatically transforms into Salamence after being put out:
HP: 300
Attack: 553
Defense: 196
Special Attack: 230
Special Defense: 196
Speed: 492
I'm unclear if eccentric copies the opponent's ability as well, or if Ditto's ability remains eccentric. If it also copies abilities, Salamence also has to deal with either Intimidate firing back on him or Ditto now having access to Overconfidence. Now you apply your own item to Ditto. Most people will run a Choice Scarf on it, so let's just say that this Ditto has one too. Now its stats look like:
HP: 300
Attack: 553
Defense: 196
Special Attack: 230
Special Defense: 196
Speed: 738 (492 + 1.5x applied from your Choice Scarf)
So now you have a Scarf Salamence with the power of a DD behind it. Now let's just play devil's advocate and say that it can copy abilities too. It gets even worse if your Sala's ability was Overconfidence and Ditto could take it, meaning that each kill nets him a 1.5x boost to his attack. A Choice Scarf user with those stats that gets 1.5x his attack power each time he kills. All with
ZERO turns of set up and no risk getting him in.
Of course if you have a Mamoswine with you, then there should be no problem at all

. Nothing's going to stop Ditto from switching out and coming back in suddenly as a Mamoswine with your Choice Band Mamoswine's power but the speed of a Choice Scarf Mamoswine all in one package.
That should about illustrate how his stats should work.