I'm using a iS137 with an 8.6m and 9.5m. I use F-Hot fins in S1 (stiffest lay up) in 52cm, 48cm and 46cm.
The 52cm and 9.5m for crappy super light 10-12 knot winds.
The 48cm gets used with the 9.5m in powered up to over powered, and 8.6m when powered up.
The 46cm get used with the 8.6m in over powered winds and its super quick make things much easier.
Every manufactures fin shape and carbon lay up feels different size for size c/w other brands. So not sure how the sizes/power etc are for the Drake ready to race, but if the 48cm is starting to feel big, I'd guess that the 46cm would do nicely.
I have heard that the new Drake's are working really well, and have heard that the Drake Down wind is especially nice. Maybe you would be better looking at a 48cm Drake Down wind as this would give you more control in over powered conditions than the Drake Ready to race 48cm.
Just my 2 cents worth