For posterity, I'll repost this here as well:
When you have a filtered stream open, the percentage of tweets you are allowed to see is determined by the overall throughput of tweets you're receiving based on your query.
If you were filtering just "flu" for one day, and all the tweets about flu made up less than 1% of all the tweets happening on Twitter at the moments you were streaming, you would receive all the tweets mentioning "flu." If lots of people were tweeting about "flu" on that day and it raised above 1% of the total firehose volume, you would get a portion of the tweets and rate limit message packets telling you how many tweets you missed.
If you were to combine the track terms "flu", "fever", "cancer", and "headache" you would increase the chance that the total volume of results you'd get back would exceed 1% of the total firehose volume. You would get rate limit messages telling you how many tweets you missed, but not necessarily which terms they had matched with.
So the answer to your question is that maybe you'll get the same amount -- depends on the volume happening in the other terms.