I converted my 15" STs to 16" LTs a while back, and that involved putting a 2" lift onto the suspension. I bought a set of LT rims and E rated tires from a guy who said that even though the rims were stamped "65 psi max" he ran them at 80 psi no problem.
The clearcoat is peeling off these rims and they look awful, so this time around I bought
new wheels and the PSI rating says "Limited to valve stem". The weight rating is more than enough to handle an E tire, and the valve stem for the TPMS I use is rated at 100 psi.
Here is the
same rim in a 15" that says the same for PSI rating. If you don't want to go to LTs, then I would upgrade the rims and get the best E-rated ST tire you can find.