I am writing to report that Instant Send is working with SouthXchange!

I have tried both instant and regular send. If you send via Instant Send, the coins are available as soon as the first block has been mined, so it's at least 6x faster than regular send. In most cases the coins will be available in 2-10 minutes with Instant Send, whereas with regular send it will be about an hour.

So I encourage everyone to use Instant Send, it will save a lot of time waiting. To use Instant Send, simply check the checkbox at the bottom of the Send tab. It will remember your selection for future sends.

Edit: It seems to be only working for transactions up to 5,000 BBP (or some specific size in bytes, I'm not sure), but the limit will be increased in a future update.