First off, nice post liftoff, it's really an eye-opener.
In terms of the "Excel spreadsheet illusion", returns aren't going to be perfectly linear and consistent. That's just not possible. The point is you're looking for a monthly average over a large sample, say 2 to 5 years, for example. But +/- 1 or even 3 standard deviations is not surprising. Rather than just say, 20% every month, and going 1.20^12 = 8.916, try using Monte Carlo analysis and look at possible standard deviations of the stats of your trading system (Profit factor, win rate, etc).
I'm more of a systematic trader (mechanical) and really it's just playing with numbers and statistics. Eventually the law of large numbers will show a profitable edge (assuming my algorithms are profitable). Sometimes my system will make 10% a month, sometimes lose 10%, sometimes make 40%, sometimes lose 20%. But in the long run it's still a positive expectancy. There's a great YT video that talks about expectations from counting cards in blackjack, and it is exactly how trading works. Except substitute "hands" with "trades". You'll see quite easily the difference between 1 trade/hand and 1 million trades/hands.
Social trading
In terms of these so-called "professional" traders, they aren't. They're the same as you or I, if not worse. Do you think Soros would start a trade copying service? It doesn't make sense. I agree with insidertrader, it's bulls**t.
Also, another point I want to make is this: as you may know, the majority of these signal services use intraday (scalping), automated strategies which are ran as EAs on their MT4 platform. Now, I'm not against automation at all, but, what I do find disturbing is the fact that they are intraday, and sometimes scalping for just a handful of pips each trade.
Some people might think this is nothing, but this is HUGE. Spreads play a huge role in a trading system. I had a trading system that I had programmed once. I backtested with 0 spread and it gave me 300% return on fixed lots, with 1 pip spread it quickly shrank to 60%, with 2 pip spread it's not a negative system. You'll find that many trading strategies are losers ONLY because of commissions, spreads and slippage. I mean, think about it, price goes up or down, it's just as hard to choose the wrong direction as it is to choose the right direction. The only thing is that you are paying spreads & slippage on every trade.
Institutional trading
In terms of comparing your returns to the large hedge funds, you may find that 10-20% a year for a firm is a lot, but there are retail traders claiming to make 100%+ a year. How does that make sense? Well, there's a couple factors. One of the main ones is leverage. Think about it: How many lots are you purchasing per trade? Assuming you had no leverage, does your current balance even have the margin required to open the position and place a stop loss?
As a retail trader, we are offered 1000:1 leverage as a maximum if you look carefully. The reason is because we are not trading with a lot of money. What prime broker can offer a multi-billion hedge fund 100:1 leverage? The fact is, no one. So a hedge fund trades with typically 1:1 whereas we usually trade with 50:1.
What does this mean?
Assume you made 100% this year with 50:1 leverage. Now, assume you were a hedge fund and instead of 50:1 you had 1:1. What this means is your return just decreased by 50 fold. Instead of 100% this year, you would have made 2%. Now that you've scaled your returns, you can accurately compare your return to institutional traders.
Hopefully this post has made some sense. I'm not the best at explaining things.
Clark