The New Success group i had were the the ones which were first generation 7 speed models carrying the painted alloy main body for the levers.
From what i can remember mine worked well with all 7 speed ShimaNO UG drivetrains, and shifted fairly decent, of course this was done using the New success rear derailleur as well.
Since the later generation 7 speed type Dura Ace rear derailleurs became available as a 7/8 speed model, i could only assume that ShimaNO thought there were some issues with early units and changed something (spring tension, limits etc), on the derailleur to accomodate for the extra gear capacity as their first 8 speed units became available.
The spacings on the 7 versus 8 for ShimaNO was very close,as long as you used the UG cogs in an 8 speed stack, HG cogs were a bit thinner so the whole stack ended up being a bit narrower when all HG cogs were set up.
This slight variation caused some cog line up issues in either end of the derailleurs travel if you tried to set it up to be perfectly aligned from one side. Needless to say, the ideal fit ended up being that you would align the middle cog and let the fuzzy areas slowly fall off center to the outside cogs, not perfect at all,but with the wide chain and the indexing fiddled with, it worked.
You could not however just buy an 8 speed cassette body for your 7 speed ShimaNO hub and change it out, since the new 8 speed units now used a different thread pitch and bore for screwing the longer cassette body into the 8 speed hub shell, you had to now change the complete hub unit as well. ShimaNO were thinking too.
It was a time when we were all much younger and things were simple yet complicated, and we all had fun riding like we still do now.