Looks like you're doing everything obvious that you can at this point. When requesting the MTU, what value are you requesting? 68? As a habit, I've always requested a higher amount than I know the device can handle (as setting the MTU is a negotiation anyway). If you're getting 69, that's coming from the device (or android is messing something up internally). I would look into the firmware a bit to see what's going on with the MTU negotiation. I haven't found any way to affect this on the phone side.
One last thing you can do to increase speed, is to lower the update rate for SweetBlue's internal update loop. The default is about 20ms. You can do this like so:
BleManagerConfig config = new BleManagerConfig();
config.autoUpdateRate = Interval.millis(1);
BleManager mgr = BleManager.get(this, config);
You can always update the config later, and set it back on the manager instance using BleManager.setConfig(). I would suggest putting the loop back to 20ms when done with the OTA.
EDIT: I also find that article highly suspect with regards to the numbers they show for android. From our experience, bluetooth has always been faster on iOS. Android may have the capacity to have quicker transfers, but in practice, it's has ALWAYS been much slower, and buggier (hence the need for SweetBlue, and also why there is no SweetBlue for iOS).