Jtr, why you would want to T/O with packs on and being supplied by the engines is a mystery. If you do it'll cost you 17500kgs off the TOW. Thus airlines either go packs off or supplied by the APU, thus there is no penalty. It is very rare even at MTOW that a flex T/O is not performed, and that is likely only in hot and or hot/high conditions where the engines flat rating is exceeded.
Probably true that you will get a derate on a -400 to a higher temp than a flex on a -300, but as MarkD has pointed out, the -600 is nearer to a competitor, and I believe you will find the comparisons here a little more hazy.