Steve,
I haven't flown a 550 in a 185, but we switched to 550's in several 206's. I think the 550 got a bad reputation early cause people were running them too hard, didn't understand hp rating or leaning on these engines. Also, the auto lean feature is a bad idea on these low level airplanes, so as da man said, stick with the stock manual system. Continental also changed the design of the cylinders several years ago, and that pretty much eliminated a lot of the cylinder problems.
I ran a 550 in a 206, almost always at 3800 pounds on floats for 1400 hours, and never touched a cylinder. Loaned the plane to the wrong pilot, who put 20 hours on it, and had to change a cylinder. He didn't understand or care about setting power, or leaning, and refused to read the book.
This is a continuous rated 300 hp engine, at 2700 rpm, whereas the 520 is a continuous rated 285 hp engine, with 300 hp permissable for 5 minutes. Percent horsepower settings for power and leaning are based on the CONTINUOUS power rating, not the full power setting. So people who are used to 520's get in the airplane now has a 550 and push up 24 square, as usual. Except that's way up there in percent power, like 80 percent. Read the book--leaning is prohibited at these very high power settings. Point is, though, you don't NEED those very high power settings to do everything the 520 did, and more.
These engines, since they are rated by the NEWER rating standards, actually put out more power at takeoff setting than ANY 520, AND they have much more torque. Finally, they are much smoother.
I'd certainly recommend a 550 to anyone who will take the time to read the operators' information on the engine and learn to run it. If this is a customer who NEVER will listen to advice or read a manual, may not be the best, but those types will screw up a 520 as well, so....
Also, this engine, at relatively low power settings, runs lean of peak so nice you can't even believe it. I ran the one in the 206 all the time at 13.2 gph or so in cruise.
MTV