I put a 18" front on my 550 and I love it. Handles great and has a lighter feel. I also went with twin leading shoe on the front. Its also seems a bit easier to find more modern tires for a 18 rim.
What did you do for a fender? The one in your pic looks to fit better than a 19" would.
I lucked out and got it off ebay. The guy selling it didn't know what it came off of, but it looked right. It was NOS in the original packaging, but had no part number. It isn't Honda, maybe Suzuki or Kawa. Anyway I cut it down to the way I liked and had to drill two holes in the mounting bracket.
One of the benefits of the front drum is it really cleans up your handle bars (no master cylinder or brake line). Still plenty of power if adjusted correctly. The only problem I would foresee would be fade when used on the track, but I have not had any issues on the street.
I also went with CB360 instrument gauges, about half the size as the stock 550 ones, so they are much more compact and tidy.