In '99 many people had huge percentage gains and diversity wasn't even needed. That time was special though.
These days, I think something on the order of 200% is possible if you concentrate on 2-3 stocks with everything else minor. It may be manageable with a $100K portfolio, or even a $200K, if a person is working. What about a million though? Do you really want to put $300K+ very often into 3 individual stocks? I think once in a while, it might be a good move to do it for one stock, provided the downside is limited and you really know the stock. That might give you a good shot at 100%, but probably not 200%. If you put large amounts in 6 of them, one is likely to blow up in your face though (See WHAIE). Anwyay, if you did make your 200%, what would you do next year, million dollar positions? :)