@Stoned Rose
Simply put, if you sign a German player at a young age or mid-way through his career from the Bundesliga then yes there's a chance you might have unearthed a rare gem (Gundogan,Rudiger, Kross,Ozil, Khedira) and then they can go on to perform into their 30s as long as they remain in that team where they are already used to the system.
But if you go on to sign an older German footballer who spent his entire career in the Bundesliga to a team in the PL, the adjustment process and physical demands would most likely be too much for that footballer.
The rare cases are those who decide to go to a much slower league like the Serie A. Podolski was below average in the PL but thrived in the Serie A, same with Klose at Lazio and Mario Gomez at Fiorentina. There's a reason why Muller turned down the chance of leaving Bayern as soon as he hit the 28 year old mark, same thing with Marco Reus at Dortmund. These players even go on to take heavy pay cuts when they hit the 28-30 year old bracket.