You have downwind backwards. A spinnaker catamaran sails both FASTER and a SHORTER DISTANCE downwind than a sloop catamaran. In order to sail the best VMG, a sloop has to sail a hotter angle than a spin-boat.

If you're going to have the sloop sail dead downwind, you'll have to slow it down by more than a factor of 1.4.

You're also postulating that a 10% sail area increase in the jib results in a 34% increase in upwind boat speed, which I don't believe is reasonable. Certainly not when you postulate that a 90% increase in sail area only increases downwind boat speed by 30%.

Think of it this way, if you could sail dead downwind without a spinnaker at speed X, then if by adding a spinnaker, you could sail at speed 1.3X, but you'd have to sail 1.4 times the distance, your VMG would be 7% slower. Why in the world would you ever put a spinnaker on the boat in the first place?

That obviously is not how catamarans sail in the real world.