Hi Steve,
1. Check your Handbook not the web.
2. if your GS450H is 2007, I would avoid large rims (19/20) and low profile (35/40/45) as they tend to give a hard and uncomfortable ride and can lead to kerbing the wheel rims.
3. My tyres are Kumho All Seasons at approx £85 each and on 225/50/17R tyres giving a smooth,quiet and comfortable ride.
Do let us all know what you decide.