I am trying to understand some of the loss mechanisms in a modern
rocket engine.

In a staged combustion cycle rocket engine, there are losses in the
turbine that drives the turbo pump/s. The friction losses in a turbine
show up in the gas as 'reheat'. Since this heat is available at the
trust chamber, it should not effect the Isp of the engine?

I know that this is the case for regeneratively cooled engines, the heat
taken out is still in the loop. So its no loss.

Am i correct? Can you get away with a very inefficient turbo-pump?

Thanks
Greg