Out of the current WSBK field, you have plenty of riders who've been in the MotoGP class, and none that has gone the other way. That could mean the best riders are not in this series, so the level of competition would be higher.
What Yamaha was looking at was both a comparison to their riders, and some form of development throughout the sessions. They were hampered by the weather at some point as I recall. The noises coming out of Yamaha however was that they were underwhelmed by Toprak's performance.
I do believe that he will be able to score points on a regular basis, out of the blocks. I can't see him being a race winner though. Not with the level MotoGP is at the moment, and with the difference between the machines. Even if you factor in the rule changes coming for 27, he'll be up against a Marquez, Bagnaia, Martin, Acosta and more. All that, after a decade in the non-prototype series.
I hope he stays and creates a legacy in WSBK. He is a brilliant draw for the series.L should have kept him for longer and got him to sort the bike for them,