I've had a Toyo Proxes on my R3 Classic for year now. I ride the R3 mainly on interstate highways and I am very pleased with the handling both in twisties and the rain (Southern MS). The only drawback I can share is that I took a wet radius at slow speed, while accelerating once and I about lost the back end-may have been road oil-but that sticks in the psyche. I don't remember what the Metzler feels like anymore.
The metz does not let you know when it is breaking loose. I went from a Honda Valkyrie to the R3 std. The higher weight distribution was a little hard to learn when it came to stopping. I found the back end coming around without any notice when running the Metz 880's. They don't squeal, ever. They just rub off and smell like rubber burning. With the Cobra's you hear it and can get a better feel for the braking.
Mezzlers were costing me ~$100 per month. Went darkside and love it. It does take some adjustment and a few hundred miles to feel normal. She felt like she was on RR tracks at Maggie Valley.