Think of it this way: The flex thrust rules require that at any point, you must be able to advance to RATED TO thrust for you actual ambient temp. So, Vmcg and Vmca should be based on rated thrust.
And even though you're only using thrust equivalent to a +48C day, the wing is still flying in the cooler actual ambient temp. So, you'll use less roll than the perf. charts for +48C would predict.