Really not sure you should be considering cleating the spinny as its the one sail that will damage both the sail and boat when things go wrong if the sail is cleated. I would suspect you will be swimming far more than you are now.

The main really is the one that should be cleated as it is has far less influence when sailing downwind than the spinny. I have found that keeping a moderate amount of sheet load on to set the main and support the mast, together with releasing the downhaul, outhaul and mast rotation ( yes you have to do all of these before putting up the spinny but they become routine eventually ) and then using the travellor to " calm " the boat down if you feel you are being over powered.

Its interesting that as part of the expermintation with Bitsa that I fitted a much smaller spinny from a Mustoo Skiff, only 14sqm but much more skiff like with a very easy to fly shape rather than the more flat spinnys we are using. Half the time to get it up and flying and so much easier to control to the point that the whole package was not much slower in mid strength winds. The boat was much easier to sail as well.