I watched the 1st half of Hamburg tonight to have a look at Meffert. The were playing a cup game against lower league opposition so take this with a fairly large pinch of salt.
He's okay/decent. He's got good size, brings a real calmness to midfield and anticipates danger well. He's solid on the ball (not a patch on Seri) but he's defensively better. The issue which stood out for me is that he lacks mobility and I think this would be a real issue. If he was on a free or cheap, I think he'd be a decent pickup but he's not the type I'd be putting £1m or Euros on the table for, especially as he'll have little resale value.
If I was being brutally honest, I think we could get better. I'm not sure he's enough of an upgrade on Simons currently and Simons can improve drastically over the next 6/12/18 months.
And I think we need 2 midfielders, one to play deeper and one that can play more advanced/box to box. If we take anyone from Hamburg, Reis was very impressive, looked a cut above.