Absolutly no sense, all is based on your strategy.
You can specialize in one instrument and squeeze it. be expert of that currency!
or you set yourself to find any good opportunity on all pairs.
Not a place to compare or say one is better than the other as this is a style of trading where people are different.
BTW nice demo backtest results :)
I would put the question like this: which is better (i) optimize an EA in several different pairs separetelly, and them use the different paramethers on each pair or (ii) optimize an multicurrency EA in several different pairs at the same time. I vote the first option.
bufflet warrant say that you should district your investment when u don't know what you are investing.
concentrate on one shape currency !
The secret is the money management