I recommend futures. 1 lot liquidty is superb. But, on news, I highly recommend that you use IB's GAT(good after time) order qualifier set to at least 5 secs after the news release time. Of course there are those special times when the news is released a little earlier or later than scheduled. But the reason I suggest this is that in each of the pairs you listed, the spread can get very wide. On the Eur FX, 15-20pips during the first few seconds. OR, simply do not set any stop loss orders until at least 10 secs after the release so you don't get bounced out simply by the spread. After the initial 5 secs, spread tightens back to 1-2 pips. You could trade on Ideal pro, but you'd have to worry about possible slippage. Or you might miss out on a trade all together is the market is moving fast and your order is a limit. Incidentally, the spread widens here too but not to the smae degree as with the futures and recovers 1 sec or two faster, back to 1 to 2 pips on Eur/USd, 3-4 GBP. Not sure about the CAD since I don't watch it. But being that it's less liquid than the other two, it's a safe bet that it does the same too.
your stratgey is not scalable. Stick to hitting the bucket shops in small sizes so you won't get noticed. Increase your size and they'll put you on manual, trade on futures and you'll have to deal with huge spreads.
You can scale at IB, but must scale larger. Future spreads on IB's forex(GLOBEX) are less than Oanda. I'm trading them now.
I am unsure why if your strategy is profitable you would want to sclae down. On IB around announcemnts the spread is uusally quite tight and sufficient volume up to at least 20m within a 5pip spread on eurusd but you'd be fooolish to think you can consume all of that. The liquidity providers will simply just reject your order. I'll agree that spreads on globex for the majors are usually 1-2 but aroudn announcements you can forget trying to get anything filled at a price and size that would be better than at a bucketshop.