Kogel sells larger ones that fit the OEM cage on the newer Shimano "R" series of RD's. I think you can go up to 12/14 on the factory cage. Like $150 on sale.
I went that route as I'm a TT nut and a half watt is a half watt more than I can train more for. Bigger reason was to go "matchy matchy" on gold chain and gold pulleys. The half a watt is whatever.