The only possible benefit from raising your WOT shift point would come immediately AFTER the upshift; you'd enter 2nd at a slightly higher RpM, than with a lower redline.
The problem is that your 'teenage' valvetrain was barely capable of upshifting by 5600 RpM BRAND NEW (and unless your 4L60E was rebuilt with a higher redline in mind, it won't like that either).
You at least need new valvesprings. While you're there, might as well throw in new guides, 1.6 rockers - maybe even pushrods and lifters?
Sounds expensive, but that'll allow the engine to both withstand and reward you for WOT upshifts at 6000 RpM … unless Brian H. tells you otherwise. In any case, your equipment and your PCM guy should tell you when your WOT upshift points are. You telling them is not good advice.