I think that YS would be right at home in the Ultimate. In Higley's book "Bipes", it discusses making a few reinforcements to the firewall and upper forward stringers to strengthen it for 1.20 application. The forward stringers (just behind the firewall on the top of the forward deck) are thin balsa sticks. He says to replace them with a larger square stock. I believe this kit also has die-cut firewall with some cut-outs in an attempt to lighten it. He suggested scrapping the cut-out piece and making the firewall out of a full piece of solid plywood. With the weight of the YS, check your equipment placement to see if you can get balance by moving your radio/battery toward the rear in case it ends up nose heavy.

Hope this helps,
Pilgrim