Got up to about 8mph downwind, but I had to reef because it felt too dangerous when I almost pitchpoled.
Good to see you out on the water at last!
Don't feel bad about reefing- that's what it's for. I would have done the same as you in those conditions, though not because I was concerned about pitchpoling as that's highly unlikely. As a guide, when the lee ama is submerging like that you reef. You will actually go faster and have less chance of breaking something.
The other option is to get out on the tramps. The TI is perfect for this and with the 'crew' hiking out the handling is greatly improved in windier conditions.
Here's a pic on a broadreach with gusts around 20knots. The sail is furled a bit but the TI is nicely balanced, even with my wife hiking out, the windward ama is mostly just out of the water.