Foot position and leg/seat room on the GL1800 has been a sore spot for many riders. I'm lucky that my legs are short so there is more then ample room there. The stock seat fits me well also (another plus)...
Foot position is another story. My joints are loaded with arthritis so having the feet on the rider pegs was very painful. I redesigned the mounts so I could flip the rider foot pegs up and rest my feet on the Aqua Shields (Motor Trike's name for running boards). I also installed a set of Kuryakan highway pegs that adjust back and down just far enough for me to reach and rest my feet in that forward position.
You might question the awkwardness of shifting and rear brake control from the alternate foot positions. NOT A PROBLEM. I'm not the most coordinated person in the world and I've become very use to it.
I don't think there is any comparison in the power you would have with the GL1800 converted. It would be my choice for sure.....