Curious as to why they would use 300 BTC for an experiment when 5 would do the trick just the same...
Because it wouldn't do the trick just the same. If they are sending transactions just a few blocks apart, then 5 BTC wouldn't be able to be sent without a fee. Remember that a fee is required whenever Bitcoin * Days is less than 1. 30 minutes * 300 BTC = 6.25 Bitcoin Days, but 30 minutes * 5 BTC = 0.104 Bitcoin Days.