Found an interesting article on high frequency FX algo trading -
http://www.fxeagle.com/HighFrequencyAutomatedFXTradingFXandMM.pdf
A little old (2004!) but the points it makes are still relevant. The issues that caught my eye were
1. The data needs to be solid, and even if it is, you can't know for certain if your order will be filled - this could dramatically alter your profit
2. There are a few large winners which make the system profitable - tdrtw, I guess this doesn't apply to you as you're operating strict profit targets?
3. The exchanges get bombarded with orders from algos. I know this is now a problem, because Interactive recently sent out a memo whereby excessive order generators would get charged (but it's a fairly high threshold)
Now, given this article was written 7 years ago and development has been significant, one wonders which timeframe the retail punter should stick to. I personally can't see any value in trying to extract below a one hour time frame using an automated system, but that might just be because I've not found anything yet.
Furthermore, I try to keep my systems as simple as possible, to avoid curve-fitting. The system I'm running with an algo at the moment has an average return of 0.08R over the last five years (approx 600 trades/year), although that number is significantly higher in the last two years. The return/drawdown ratio is around 0.8, with Sharpe of about 0.75. I suspect the banks are operating at much higher rates than that.