@Viridian Hawk, I believe quantopian accounts for bid-ask spread with their slippage model - which was on during all backtests. Either way, placing a limit order would "subvert" that problem - you'd just have a lot of open orders. This algorithm closes all orders before each transaction-causing function.
@Peter Bakker. I'll be making a post on this, but to answer your question - Sentiment data, as useful as it is, is flawed. Why? because humans are involved. Sentiment sourced from media is plagued (quite significantly) by many things. One of the most important is something that lies within all media, regardless of the language: Positivity Bias (known as Pollyannism) - the tendency for people to remember events in a more positive light (this happens at the subconscious level). Equally important is negativity bias which happens at the conscious level.
When a person says they think a stock will perform well, we must be concerned about their previous experience with that stock... obviously we can't look at this with the data available - We can't even determine if this person is just saying something just to say something...
To get around this, we need to get into sociology, to keep this post short, I am mostly concerned with the consequences of the hive mind (or herd mentality), and information loss (lack of fact checking).
Q1500? Nope, the entire universe of stocks.