Besiktas loaned Aboubakar from Porto and Talisca from Benfica, both had little time in their own teams and got loads of time at Besiktas. Both deals went smoothly and can go as well for Everton if they bother to read the contracts they sign. Don't know about that infamous Yobo deal, but I doubt Besiktas, having an idea to sell players to pay off debt, would dare to do some shady business against a Premier League team. Having an easy couple of millions would result in losing tens.
If Everton thinks Klaassen will be useful, he should definetely stay. Otherwise, having a player actually taking time and raising his value again looks like the best option. Besiktas might not be able to buy him in the end of the loan contract but Klaassen playing full season would be an easy sell for Everton for a 10 million minimum.