When I had my Windjammer, I went from load "C" to "D" tires. I had the same question as the OP. I contacted LionsHead the supplier of my rims. They told me that my rims were rated for 50 psi, but if I went to a metal valve stem, I could go up to 80 psi. The rubber valve stems was the reason for the 50 psi limit. I had the fancy 15" aluminum rims.
Russ & Kim
2014 Alpine 3600RS
2007 Dodge Ram 3500 5.9 4X4
2008 Harley Davidson Road King 105th Anniversary
(2011-5 bad year)(2012-40)(2013-45)(2014-23)