I could easily see such a roster beating 5-10
Not overestimating Mantuu at all. Mechanically he's as good as any AWPer excluding S1mple, ZywOo, Broky and you could argue m0nesy but he's a bit of a headless chicken in comparison to alot of the top team's AWPers.
Even in this Nexa lead iteration of OG Mantuu can still find frags quite easily he just struggles to find impact early which you can attribute partly to Nexa's extremely loose and basic calling style. It's clear to me that Mantuu is greater than the sum of the current team.
Top 10 based on having a reasonably well built team in theory, YEKINDAR adding extra depth to OG's very lacking tactics right now as a mid round caller and JKS is just a straight upgrade to current niko or valde.
I don't see this roster beating FaZe, NaVi, ENCE, cloud9 or Heroic but absolutely could take maps and matches from NiP downwards, even cloud9 is debatable. In terms of sheer firepower also I think you overestimate everyone not named FaZe, NaVi, ENCE, cloud9 and G2.
YEKINDAR jks Mantuu is a solid core with YEKINDAR obviously being the standout but the difference is Nexa's contribution to a team is nothing to do with his IGLing (he barely calls at all) but rather the fact that he's an above average fragger. You have a fragging IGL, a good passive star rifler in jks, an AWPer with solid potential in Mantuu and an intelligent aggressive star rifler in YEKINDAR. FlameZ is the weaker link but you need someone to play like a Qikert or Buster for YEKINDAR to entry off of.