The fact that they didn't run off with your 20 BTC might be evidence of their legitimacy. But if they are running a ponzi and they are smart they would have happily returned your 20 BTC because they anticipated you doing this:
I put 5 BTC back into my btc-arbs.com account and will prob bulk up to 50 btc and leave for the month of March.
Keep in mind that Trendon Shavers ran a Ponzi scheme for months and collected millions of dollars worth of bitcoins, and Bernie Madoff's Ponzi scheme ran for years and collected billions of dollars.
Also, keep in mind that there are hundreds of people looking at Bitcoin exchange arbitrage. If there is any profit from arbitrage, it is going to be small.