The BT45s are a great tire. Better feel, traction, and handling than the Avon AM26s that came on the bike, especially at high lean angles. I don't think you can get them in 19" sizes for the G5.
What are the stock G5 tire sizes? Something to consider would be the new Continental Classic Attack if you can get them in appropriate sizes for the bike. These are tires designed by Conti in vintage sizes but they're radials, not bias ply. Radials are far more flexible and generally handle better than bias ply tires.
Barenekd had some Pirelli Sport Demons on his G5 and loved them, and he rode very hard.
For me, I usually look for a sport touring tire with good wet weather characteristics since I live in the Pacific Northwest. The BT45 was what I could get in the right sizes for my bike but I really like sport touring tires from Michelin. They wear well and predictably, have good dry weather traction, and amazing wet weather traction. If you can get a set of Michelins that fit I say go for it.
Scott