Its more worth it to use the berry before lvl 100. So they can gain the EV points you took away with the berry in another stat. I also heard of some Macho Braces you can win in battle tower. That are specific to each stat. I forget the name of them though. But there are 6 macho braces for each stat if you wanna raise that stat the most. Im going to try to get my AR today.
It has to be noted that those items are called the Power Items. They give you 4 EV in a certain stat when gaining experience., no matter which Pokemon you fight. The original EV(s) you would get are still included with the prize also =)
Be aware that Pokerus ALSO effects the EV you gain from those Power Items, meaning they will give you 8 EV after each time you get experience. Say, you're up against a Magikarp. A normal Magikarp gives you 1 EV point in SPEED when you defeat it. Equip you Pokémon with the Speed Power Item (don't know the real name) and it gets 5 EV for every Magikarp. Give it Pokerus as well and your Poke will get 10 EV for each Magikarp (note the huge difference between 1 and 10). The normal EV you get (1) and the Power item EV (4) have both been doubled.
At the same time, a Pokemon like Graveler who gives you 2 EV in Defense will give you 4 with Pokerus and 12 with Pokerus and the Defense Power Item ( (2 x 2) + (4 x 2) = 12, go figure).
Power Items also give away a better way of EV training, not just the ridiculous EV boosts. As I have told you before, a Power Item gives you 4 (or 8 with Pokerus) EV in a certain stat, NO MATTER WHICH kind of Pokemon you defeat. So say you wat to train your Pokemon in both Attack and Sp Atk, you could equip your Pokemon with the Attack Power Item, and defeat monsters that give you Sp Atk, like Haunter. With Pokerus, that would give you 8 Attack EV and 4 Sp Atk EV at the same time, opening up a good way to train multiple stats at once.
This whole process with Macho Brace works exactly the same as the above, except that Macho Brace double the EV you RECIEVE FROM THE DEFEATED POKEMON, unlike the Power Items that give let you recieve EV from the item itself. Looking at this, there's not really a use for Macho Brace anymore, I think, as training with the Power Items is waaaayyy faster.
Simply put, EV training has never been so easy before, so make use of that.
About lvl 100 Pokemon: how are you planning to Re-EV train a lvl 100 Pokemon who CAN'T GET ANY MORE EXPERIENCE. EV comes with experience, you should know by now ;p