Some of the best players who DON'T have a positive record:
Wilander (exactly .5), Safin, Hewitt, Stich, Edberg, Nastase, Courier, Rafter, Kafelnikov
Nastase is a special case because the rankings system was only created in the end of 1973, and he had his best years before.
Wilander it would be interesting to see how he rated until the end of 1988 or 1989, because afterwards he was the shadow of what he was before.
I'm most surprised about Edberg.
But generally speaking, I'm not that fond of that ratio against top-10 because some players can be in the top-10 but play badly, and they will only fall from it later ... whereas they may have been better BEFORE entering top-10.
Amirbachar may have ideas on that topic as he also created some specific complicated indexes.
Personally I don't look at ratios against top-10 but a list of 30 players I consider as very good. For instance for long Hewitt was not in the top-10 but still it was harder to beat him than another player who was top-10 ... when he did actually play.
Some players like him or Haas or Baghdatis or even Wawrinka have better stats against best players than their current ranking might let it look.
As for Raonic, his ratio against best players in my stats is comparable to Isner's then I'm not sure exactly what to derive from this for his future : I also think basically that such big servers will tend to be more dangerous for top-players but be more vulnerable against other players ... and that's especially a problem for them in slams (also because they may have longer matches)