hard 2-3 shift and ? slippage -IPMO

I have a 727 mrvb that under light load shifts with quit an impact on the 2-3 shift. I also have rpms at the stripe that are ±300 higher than last year.
The TC has been disassembled and checked - nothing wrong with it and my calculated slippage last year was in the 5.5% range.
This year with a complete new performance built 727 and the same TC the slippage calculates out to ±13%. we have even changed cases:angry3:
Anyone got any suggestions/corrections? miracles?
Can the tranny be slipping in 3rd gear? and how would I know? line pressure too low for this kind of build?
thanks