It sounds like the best correlation to make is that the RSI data can be used to demonstrate the differences in how various strings lose tension compared to each other, rather than trying to use those numbers as absolutes as to what should be seen (numerically) in a given frame.
Agreed. It's not the specific numbers that matter when looking at RSI or TWU data, but comparisons and relative differences or trends between strings can be better understood with the accompanying data found at RSI and TWU.
For instance, as a whole, Natural gut always have the lowest tension loss numbers and stiffness, and are all fairly close together. Then you have synthetics, multis, co-poly and finally kevlar; each group increasing in stiffness and tension loss, respectively and in most cases (the occasional stiff multi or soft poly or whatever can be seen as well). There are other trends one can browse, like stiffness, and then all sorts of other relationships on TWU, like dwell time, energy return, and so on.
I agree the tension loss stats are the least useful on these two databases, although they do generally show that natural gut, for instance, has better tension hold than poly. But whether "string X loses 17 pounds" versus "string Y loses 18 pounds", that hardly will matter on court. I also agree that a StringBed of interwoven strings behaves way differently than a single strand, which applies also to stiffness, dwell time, etc. The racquet you put a string in also changes the way a string behaves. There are strings I love in the dense, flexy PT280 but can't stand in the open, stiffer YTPP. Yet both of the racquets I have setup to the same weight and balance, swingweight, etc. Strings also remain playable longer in the denser, flexy racquet; and breaking much faster in the open pattern.
Getting too fixated on the exact pound or percentage loss of a string isn't that important. The importance in the RSI and TWU tables, to me at least, is to see how various strings compare to each other. This can give people a good starting point in trying out a new string or finding something to compare to their favorite string. It's definitely not the 'end all, be all' but can be a useful guide.
In the end, on court testing will always be the best guide because only then can one Experience the characteristics of a string or racquet or whatever; and while subject to subjectivity or other things that can't be displayed on a graph, experiencing different strings and racquet combos tend to be very subjective; one man's trash is another's treasure and that does go a long way on the tennis court.
